diff --git a/tests/bindings/googlepb/test_vs_proto2.cc b/tests/bindings/googlepb/test_vs_proto2.cc
index c89339ae6b..49dfe49c05 100644
--- a/tests/bindings/googlepb/test_vs_proto2.cc
+++ b/tests/bindings/googlepb/test_vs_proto2.cc
@@ -40,8 +40,8 @@ const unsigned char message2_data[] = {
 void compare_metadata(const google::protobuf::Descriptor* d,
                       const upb::MessageDef *upb_md) {
   ASSERT(d->field_count() == upb_md->field_count());
-  for (upb::MessageDef::const_iterator i = upb_md->begin(); i != upb_md->end();
-       ++i) {
+  for (upb::MessageDef::const_field_iterator i = upb_md->field_begin();
+       i != upb_md->field_end(); ++i) {
     const upb::FieldDef* upb_f = *i;
     const google::protobuf::FieldDescriptor *proto2_f =
         d->FindFieldByNumber(upb_f->number());
diff --git a/tests/test_cpp.cc b/tests/test_cpp.cc
index 55bb4b390b..71f354ceaa 100644
--- a/tests/test_cpp.cc
+++ b/tests/test_cpp.cc
@@ -12,6 +12,7 @@
 
 #include <iostream>
 #include <set>
+#include <sstream>
 
 #include "upb/def.h"
 #include "upb/descriptor/reader.h"
@@ -164,7 +165,7 @@ static void TestSymbolTable(const char *descriptor_file) {
 #ifdef UPB_CXX11
   // Test range-based for.
   std::set<const upb::FieldDef*> fielddefs;
-  for (const upb::FieldDef* f : *md.get()) {
+  for (const upb::FieldDef* f : md.get()->fields()) {
     AssertInsert(&fielddefs, f);
     ASSERT(f->containing_type() == md.get());
   }
@@ -1117,6 +1118,103 @@ void TestHandlerDataDestruction() {
   ASSERT(x == 0);
 }
 
+void TestOneofs() {
+  upb::Status status;
+  upb::reffed_ptr<upb::MessageDef> md(upb::MessageDef::New());
+  upb::reffed_ptr<upb::OneofDef> o(upb::OneofDef::New());
+
+  o->set_name("test_oneof", &status);
+  ASSERT(status.ok());
+
+  for (int i = 0; i < 5; i++) {
+    std::ostringstream fieldname;
+    fieldname << "field_" << i;
+    upb::reffed_ptr<upb::FieldDef> f(upb::FieldDef::New());
+    f->set_name(fieldname.str(), &status);
+    ASSERT(status.ok());
+    f->set_type(UPB_TYPE_INT32);
+    f->set_number(i + 1, &status);
+    ASSERT(status.ok());
+    f->set_label(UPB_LABEL_OPTIONAL);
+
+    o->AddField(f.get(), &status);
+    ASSERT(status.ok());
+  }
+
+  md->AddOneof(o.get(), &status);
+  ASSERT(status.ok());
+
+  int field_count = 0;
+  for (upb::OneofDef::iterator it = o->begin(); it != o->end(); ++it) {
+    upb::FieldDef* f = *it;
+    ASSERT(f->type() == UPB_TYPE_INT32);
+    field_count++;
+  }
+  ASSERT(field_count == 5);
+
+  upb::MessageDef::oneof_iterator msg_it = md->oneof_begin();
+  ASSERT(msg_it != md->oneof_end());
+  ASSERT((*msg_it) == o.get());
+
+#ifdef UPB_CXX11
+  // Test range-based for on both fields and oneofs (with the iterator adaptor).
+  field_count = 0;
+  for (auto* field : md->fields()) {
+    UPB_UNUSED(field);
+    field_count++;
+  }
+  ASSERT(field_count == 5);
+
+  int oneof_count = 0;
+  for (auto* oneof : md->oneofs()) {
+    UPB_UNUSED(oneof);
+    oneof_count++;
+  }
+  ASSERT(oneof_count == 1);
+#endif  //  UPB_CXX11
+
+  // Test that we can add a new field to the oneof and that it becomes a member
+  // of the msgdef as well.
+  upb::reffed_ptr<upb::FieldDef> newf(upb::FieldDef::New());
+  newf->set_name("new_field_10", &status);
+  ASSERT(status.ok());
+  newf->set_number(10, &status);
+  ASSERT(status.ok());
+  newf->set_label(UPB_LABEL_OPTIONAL);
+  newf->set_type(UPB_TYPE_INT32);
+  o->AddField(newf.get(), &status);
+  ASSERT(status.ok());
+  ASSERT(newf->containing_type() == md.get());
+
+  // Test that we can add a new field to the msgdef first and then to the oneof.
+  upb::reffed_ptr<upb::FieldDef> newf2(upb::FieldDef::New());
+  newf2->set_name("new_field_11", &status);
+  ASSERT(status.ok());
+  newf2->set_number(11, &status);
+  ASSERT(status.ok());
+  newf2->set_label(UPB_LABEL_OPTIONAL);
+  newf2->set_type(UPB_TYPE_INT32);
+  md->AddField(newf2.get(), &status);
+  ASSERT(status.ok());
+  o->AddField(newf2.get(), &status);
+  ASSERT(status.ok());
+  ASSERT(newf2->containing_oneof() == o.get());
+
+  // Test that we cannot add REQUIRED or REPEATED fields to the oneof.
+  upb::reffed_ptr<upb::FieldDef> newf3(upb::FieldDef::New());
+  newf3->set_name("new_field_12", &status);
+  ASSERT(status.ok());
+  newf3->set_number(12, &status);
+  ASSERT(status.ok());
+  newf3->set_label(UPB_LABEL_REQUIRED);
+  newf3->set_type(UPB_TYPE_INT32);
+  o->AddField(newf3.get(), &status);
+  ASSERT(!status.ok());
+  newf->set_label(UPB_LABEL_REPEATED);
+  o->AddField(newf3.get(), &status);
+  ASSERT(!status.ok());
+}
+
 extern "C" {
 
 int run_tests(int argc, char *argv[]) {
@@ -1173,6 +1271,8 @@ int run_tests(int argc, char *argv[]) {
 
   TestHandlerDataDestruction();
 
+  TestOneofs();
+
   return 0;
 }
 
diff --git a/tests/test_def.c b/tests/test_def.c
index dcf80c05ec..b85b0317d6 100644
--- a/tests/test_def.c
+++ b/tests/test_def.c
@@ -189,7 +189,9 @@ static upb_fielddef *newfield(
   ASSERT(upb_fielddef_setnumber(f, num, NULL));
   upb_fielddef_settype(f, type);
   upb_fielddef_setlabel(f, label);
-  ASSERT(upb_fielddef_setsubdefname(f, type_name, NULL));
+  if (type_name) {
+    ASSERT(upb_fielddef_setsubdefname(f, type_name, NULL));
+  }
   return f;
 }
 
@@ -342,6 +344,58 @@ static void test_descriptor_flags() {
   upb_msgdef_unref(m2, &m2);
 }
 
+static void test_oneofs() {
+  upb_status s = UPB_STATUS_INIT;
+  bool ok = true;
+
+  upb_symtab *symtab = upb_symtab_new(&symtab);
+  ASSERT(symtab != NULL);
+
+  // Create a test message for fields to refer to.
+  upb_msgdef *subm = upb_msgdef_newnamed("SubMessage", &symtab);
+  upb_msgdef_addfield(subm, newfield("field1", 1, UPB_TYPE_INT32,
+                                     UPB_LABEL_OPTIONAL, NULL, &symtab),
+                      &symtab, NULL);
+  upb_def *subm_defs[] = {UPB_UPCAST(subm)};
+  ASSERT_STATUS(upb_symtab_add(symtab, subm_defs, 1, &symtab, &s), &s);
+
+  upb_msgdef *m = upb_msgdef_newnamed("TestMessage", &symtab);
+  ASSERT(upb_msgdef_numoneofs(m) == 0);
+
+  upb_oneofdef *o = upb_oneofdef_new(&o);
+  ASSERT(upb_oneofdef_numfields(o) == 0);
+  ASSERT(upb_oneofdef_name(o) == NULL);
+
+  ok = upb_oneofdef_setname(o, "test_oneof", &s);
+  ASSERT_STATUS(ok, &s);
+
+  ok = upb_oneofdef_addfield(o, newfield("field1", 1, UPB_TYPE_INT32,
+                                         UPB_LABEL_OPTIONAL, NULL, &symtab),
+                             &symtab, NULL);
+  ASSERT_STATUS(ok, &s);
+  ok = upb_oneofdef_addfield(o, newfield("field2", 2, UPB_TYPE_MESSAGE,
+                                         UPB_LABEL_OPTIONAL, ".SubMessage",
+                                         &symtab),
+                             &symtab, NULL);
+  ASSERT_STATUS(ok, &s);
+
+  ok = upb_msgdef_addoneof(m, o, NULL, &s);
+  ASSERT_STATUS(ok, &s);
+
+  upb_def *defs[] = {UPB_UPCAST(m)};
+  ASSERT_STATUS(upb_symtab_add(symtab, defs, 1, &symtab, &s), &s);
+
+  ASSERT(upb_msgdef_numoneofs(m) == 1);
+  const upb_oneofdef *lookup_o = upb_msgdef_ntooz(m, "test_oneof");
+  ASSERT(lookup_o == o);
+
+  const upb_fielddef *lookup_field = upb_oneofdef_ntofz(o, "field1");
+  ASSERT(lookup_field != NULL && upb_fielddef_number(lookup_field) == 1);
+
+  upb_symtab_unref(symtab, &symtab);
+  upb_oneofdef_unref(o, &o);
+}
+
 int run_tests(int argc, char *argv[]) {
   if (argc < 2) {
     fprintf(stderr, "Usage: test_def <test.proto.pb>\n");
@@ -358,5 +412,6 @@ int run_tests(int argc, char *argv[]) {
   test_partial_freeze();
   test_noreftracking();
   test_descriptor_flags();
+  test_oneofs();
   return 0;
 }
diff --git a/upb/bindings/googlepb/bridge.cc b/upb/bindings/googlepb/bridge.cc
index 6ae8868e4f..2a2bbbfc4d 100644
--- a/upb/bindings/googlepb/bridge.cc
+++ b/upb/bindings/googlepb/bridge.cc
@@ -246,7 +246,8 @@ const Handlers* CodeCache::GetMaybeUnfrozenWriteHandlers(
   to_freeze_.push_back(h);
   const goog::Descriptor* d = m.GetDescriptor();
 
-  for (upb::MessageDef::const_iterator i = md->begin(); i != md->end(); ++i) {
+  for (upb::MessageDef::const_field_iterator i = md->field_begin();
+       i != md->field_end(); ++i) {
     const FieldDef* upb_f = *i;
 
     const goog::FieldDescriptor* proto2_f =
diff --git a/upb/bindings/lua/upb.c b/upb/bindings/lua/upb.c
index 17fc0a865c..5ad0235cec 100644
--- a/upb/bindings/lua/upb.c
+++ b/upb/bindings/lua/upb.c
@@ -139,7 +139,8 @@ bool lupb_openlib(lua_State *L, void *ptr, const char *name,
 
 // Pushes a new userdata with the given metatable and ensures that it has a
 // uservalue.
-static void *newudata_with_userval(lua_State *L, size_t size, const char *type) {
+static void *newudata_with_userval(lua_State *L, size_t size,
+                                   const char *type) {
   void *ret = lua_newuserdata(L, size);
 
   // Set metatable.
@@ -952,17 +953,17 @@ static int lupb_msgdef_field(lua_State *L) {
 }
 
 static int lupb_msgiter_next(lua_State *L) {
-  upb_msg_iter *i = lua_touserdata(L, lua_upvalueindex(1));
-  if (upb_msg_done(i)) return 0;
+  upb_msg_field_iter *i = lua_touserdata(L, lua_upvalueindex(1));
+  if (upb_msg_field_done(i)) return 0;
   lupb_def_pushwrapper(L, UPB_UPCAST(upb_msg_iter_field(i)), NULL);
-  upb_msg_next(i);
+  upb_msg_field_next(i);
   return 1;
 }
 
 static int lupb_msgdef_fields(lua_State *L) {
   const upb_msgdef *m = lupb_msgdef_check(L, 1);
-  upb_msg_iter *i = lua_newuserdata(L, sizeof(upb_msg_iter));
-  upb_msg_begin(i, m);
+  upb_msg_field_iter *i = lua_newuserdata(L, sizeof(upb_msg_field_iter));
+  upb_msg_field_begin(i, m);
   // Need to guarantee that the msgdef outlives the iter.
   lua_pushvalue(L, 1);
   lua_pushcclosure(L, &lupb_msgiter_next, 2);
@@ -1416,8 +1417,10 @@ static lupb_msgdef *lupb_msg_assignoffsets(lua_State *L, int narg) {
   size_t userval_idx = 1;
 
   // Assign offsets.
-  upb_msg_iter i;
-  for (upb_msg_begin(&i, lmd->md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for (upb_msg_field_begin(&i, lmd->md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     if (in_userval(f)) {
       offsets[upb_fielddef_index(f)] = userval_idx++;
@@ -1442,7 +1445,9 @@ static lupb_msgdef *lupb_msg_assignoffsets(lua_State *L, int narg) {
   lua_newtable(L);  // This will be our userval.
 
   int idx = 1;
-  for (upb_msg_begin(&i, lmd->md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  for (upb_msg_field_begin(&i, lmd->md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     if (upb_fielddef_type(f) == UPB_TYPE_MESSAGE) {
       bool created = lupb_def_pushwrapper(L, upb_fielddef_subdef(f), NULL);
@@ -1660,9 +1665,9 @@ void callback(const void *closure, upb_handlers *h) {
   lua_State *L = (lua_State*)closure;
   lupb_def_pushwrapper(L, UPB_UPCAST(upb_handlers_msgdef(h)), NULL);
   lupb_msgdef *lmd = lupb_msg_assignoffsets(L, -1);
-  upb_msg_iter i;
-  upb_msg_begin(&i, upb_handlers_msgdef(h));
-  for (; !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  upb_msg_field_begin(&i, upb_handlers_msgdef(h));
+  for (; !upb_msg_field_done(&i); upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     int hasbit = upb_fielddef_index(f);
     uint16_t ofs = lmd->field_offsets[upb_fielddef_index(f)];
diff --git a/upb/bindings/python/upb.c b/upb/bindings/python/upb.c
index 497074b5c9..6cfc8e9a0d 100644
--- a/upb/bindings/python/upb.c
+++ b/upb/bindings/python/upb.c
@@ -298,7 +298,8 @@ static PyObject *PyUpb_MessageDef_new(PyTypeObject *subtype,
 
 static PyObject *PyUpb_MessageDef_add_fields(PyObject *o, PyObject *args);
 
-static int PyUpb_MessageDef_init(PyObject *self, PyObject *args, PyObject *kwds) {
+static int PyUpb_MessageDef_init(
+    PyObject *self, PyObject *args, PyObject *kwds) {
   if (!kwds) return 0;
   PyObject *key, *value;
   Py_ssize_t pos = 0;
@@ -323,7 +324,8 @@ static PyObject *PyUpb_MessageDef_getattro(PyObject *obj, PyObject *attr_name) {
   return PyObject_GenericGetAttr(obj, attr_name);
 }
 
-static int PyUpb_MessageDef_setattro(PyObject *o, PyObject *key, PyObject *val) {
+static int PyUpb_MessageDef_setattro(
+    PyObject *o, PyObject *key, PyObject *val) {
   upb_msgdef *m = Check_MessageDef(o, -1);
   if (!upb_def_ismutable(UPB_UPCAST(m))) {
     PyErr_SetString(PyExc_TypeError, "MessageDef is not mutable.");
@@ -343,9 +345,11 @@ static int PyUpb_MessageDef_setattro(PyObject *o, PyObject *key, PyObject *val)
 static PyObject *PyUpb_MessageDef_fields(PyObject *obj, PyObject *args) {
   upb_msgdef *m = Check_MessageDef(obj, NULL);
   PyObject *ret = PyList_New(0);
-  upb_msg_iter i;
-  for(i = upb_msg_begin(m); !upb_msg_done(i); i = upb_msg_next(m, i)) {
-    upb_fielddef *f = upb_msg_iter_field(i);
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, m);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&ii)) {
+    upb_fielddef *f = upb_msg_iter_field(&i);
     PyList_Append(ret, PyUpb_FieldDef_GetOrCreate(f));
   }
   return ret;
@@ -374,9 +378,12 @@ static PyObject *PyUpb_MessageDef_add_field(PyObject *o, PyObject *field) {
 }
 
 static PyMethodDef PyUpb_MessageDef_methods[] = {
-  {"add_field", &PyUpb_MessageDef_add_field, METH_O, "Adds a list of fields."},
-  {"add_fields", &PyUpb_MessageDef_add_fields, METH_O, "Adds a list of fields."},
-  {"fields", &PyUpb_MessageDef_fields, METH_NOARGS, "Returns list of fields."},
+  {"add_field", &PyUpb_MessageDef_add_field, METH_O,
+    "Adds a list of fields."},
+  {"add_fields", &PyUpb_MessageDef_add_fields, METH_O,
+    "Adds a list of fields."},
+  {"fields", &PyUpb_MessageDef_fields, METH_NOARGS,
+    "Returns list of fields."},
   {NULL, NULL}
 };
 
@@ -448,7 +455,8 @@ static PyObject *PyUpb_SymbolTable_new(PyTypeObject *subtype,
   return PyUpb_ObjCacheGet(upb_symtab_new(), subtype);
 }
 
-static int PyUpb_SymbolTable_init(PyObject *self, PyObject *args, PyObject *kwds) {
+static int PyUpb_SymbolTable_init(
+    PyObject *self, PyObject *args, PyObject *kwds) {
   return 0;
 }
 
@@ -475,8 +483,10 @@ static PyObject *PyUpb_SymbolTable_add_defs(PyObject *o, PyObject *defs) {
     cdefs[i++] = def;
     upb_msgdef *md = upb_dyncast_msgdef(def);
     if (!md) continue;
-    upb_msg_iter j;
-    for(j = upb_msg_begin(md); !upb_msg_done(j); j = upb_msg_next(md, j)) {
+    upb_msg_field_iter j;
+    for(upb_msg_field_begin(&j, md);
+        !upb_msg_field_done(&j);
+        upb_msg_field_next(&j)) {
       upb_fielddef *f = upb_msg_iter_field(j);
       upb_fielddef_setaccessor(f, PyUpb_AccessorForField(f));
     }
@@ -601,7 +611,8 @@ static upb_sflow_t PyUpb_Message_StartSubmessage(void *m, upb_value fval) {
   return UPB_CONTINUE_WITH(*submsg);
 }
 
-static upb_sflow_t PyUpb_Message_StartRepeatedSubmessage(void *a, upb_value fval) {
+static upb_sflow_t PyUpb_Message_StartRepeatedSubmessage(
+    void *a, upb_value fval) {
   (void)fval;
   PyObject **elem = upb_stdarray_append(a, sizeof(void*));
   PyTypeObject *type = ((PyUpb_MessageType*)Py_TYPE(a))->alt_type;
@@ -609,7 +620,8 @@ static upb_sflow_t PyUpb_Message_StartRepeatedSubmessage(void *a, upb_value fval
   return UPB_CONTINUE_WITH(*elem);
 }
 
-static upb_flow_t PyUpb_Message_StringValue(void *m, upb_value fval, upb_value val) {
+static upb_flow_t PyUpb_Message_StringValue(
+    void *m, upb_value fval, upb_value val) {
   PyObject **str = PyUpb_Accessor_GetPtr(m, fval);
   if (*str) { Py_DECREF(*str); }
   *str = PyString_FromStringAndSize(NULL, upb_value_getstrref(val)->len);
@@ -618,7 +630,8 @@ static upb_flow_t PyUpb_Message_StringValue(void *m, upb_value fval, upb_value v
   return UPB_CONTINUE;
 }
 
-static upb_flow_t PyUpb_Message_AppendStringValue(void *a, upb_value fval, upb_value val) {
+static upb_flow_t PyUpb_Message_AppendStringValue(
+    void *a, upb_value fval, upb_value val) {
   (void)fval;
   PyObject **elem = upb_stdarray_append(a, sizeof(void*));
   *elem = PyString_FromStringAndSize(NULL, upb_value_getstrref(val)->len);
diff --git a/upb/bindings/ruby/upb.c b/upb/bindings/ruby/upb.c
index 2817a158f7..961836692d 100644
--- a/upb/bindings/ruby/upb.c
+++ b/upb/bindings/ruby/upb.c
@@ -273,9 +273,11 @@ static size_t rupb_sizeof(const upb_fielddef *f) {
 static void assign_offsets(rb_msglayout *layout, const upb_msgdef *md) {
   layout->field_offsets = ALLOC_N(uint32_t, upb_msgdef_numfields(md));
   size_t ofs = 0;
-  upb_msg_iter i;
+  upb_msg_field_iter i;
 
-  for (upb_msg_begin(&i, md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  for (upb_msg_field_begin(&i, md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
     size_t field_size = rupb_sizeof(f);
 
@@ -301,8 +303,10 @@ static void make_prototype(rb_msglayout *layout, const upb_msgdef *md) {
   // more specific initialization.
   memset(prototype, 0, layout->size);
 
-  upb_msg_iter i;
-  for (upb_msg_begin(&i, md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for (upb_msg_field_begin(&i, md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
     if (is_ruby_value(f)) {
       size_t ofs = layout->field_offsets[upb_fielddef_index(f)];
@@ -373,8 +377,10 @@ static void msgdef_mark(void *_rmd) {
   rb_gc_mark(rmd->klass);
 
   // Mark all submessage types.
-  upb_msg_iter i;
-  for (upb_msg_begin(&i, rmd->md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for (upb_msg_field_begin(&i, rmd->md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     if (upb_fielddef_issubmsg(f)) {
       // If we were trying to be more aggressively lazy, the submessage might
@@ -495,8 +501,10 @@ static void msg_mark(void *p) {
 
   // We need to mark all references to other Ruby values: strings, arrays, and
   // submessages that we point to.
-  upb_msg_iter i;
-  for (upb_msg_begin(&i, rmd->md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for (upb_msg_field_begin(&i, rmd->md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     if (is_ruby_value(f)) {
       size_t ofs = rmd->layout.field_offsets[upb_fielddef_index(f)];
@@ -903,7 +911,8 @@ static void *submsg_handler(void *closure, const void *hd) {
   const submsg_handlerdata_t *submsgdata = hd;
 
   if (DEREF(msg, submsgdata->ofs, VALUE) == Qnil) {
-    DEREF(msg, submsgdata->ofs, VALUE) = msg_new(msgdef_getwrapper(submsgdata->md));
+    DEREF(msg, submsgdata->ofs, VALUE) =
+        msg_new(msgdef_getwrapper(submsgdata->md));
   }
 
   VALUE submsg = DEREF(msg, submsgdata->ofs, VALUE);
@@ -912,9 +921,11 @@ static void *submsg_handler(void *closure, const void *hd) {
 
 static void add_handlers_for_message(const void *closure, upb_handlers *h) {
   const rupb_MessageDef *rmd = get_rbmsgdef(upb_handlers_msgdef(h));
-  upb_msg_iter i;
+  upb_msg_field_iter i;
 
-  for (upb_msg_begin(&i, rmd->md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  for (upb_msg_field_begin(&i, rmd->md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
     size_t ofs = rmd->layout.field_offsets[upb_fielddef_index(f)];
 
@@ -1085,8 +1096,10 @@ static void putmsg(rupb_Message *msg, const rupb_MessageDef *rmd,
                    upb_sink *sink) {
   upb_sink_startmsg(sink);
 
-  upb_msg_iter i;
-  for (upb_msg_begin(&i, rmd->md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for (upb_msg_field_begin(&i, rmd->md);
+       !upb_msg_field_done(&i);
+       upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     uint32_t ofs = rmd->layout.field_offsets[upb_fielddef_index(f)];
 
diff --git a/upb/def.c b/upb/def.c
index 349dfd5b3f..3dba2215dc 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -248,10 +248,12 @@ static bool assign_msg_indices(upb_msgdef *m, upb_status *s) {
   upb_fielddef **fields = malloc(n * sizeof(*fields));
   if (!fields) return false;
 
-  upb_msg_iter j;
+  upb_msg_field_iter j;
   int i;
   m->submsg_field_count = 0;
-  for(i = 0, upb_msg_begin(&j, m); !upb_msg_done(&j); upb_msg_next(&j), i++) {
+  for(i = 0, upb_msg_field_begin(&j, m);
+      !upb_msg_field_done(&j);
+      upb_msg_field_next(&j), i++) {
     upb_fielddef *f = upb_msg_iter_field(&j);
     assert(f->msg.def == m);
     if (!upb_validate_field(f, s)) {
@@ -287,7 +289,9 @@ static bool assign_msg_indices(upb_msgdef *m, upb_status *s) {
   upb_selector_t sel;
   upb_inttable_insert(&t, UPB_STARTMSG_SELECTOR, v);
   upb_inttable_insert(&t, UPB_ENDMSG_SELECTOR, v);
-  for(upb_msg_begin(&j, m); !upb_msg_done(&j); upb_msg_next(&j)) {
+  for(upb_msg_field_begin(&j, m);
+      !upb_msg_field_done(&j);
+      upb_msg_field_next(&j)) {
     upb_fielddef *f = upb_msg_iter_field(&j);
     // These calls will assert-fail in upb_table if the value already exists.
     TRY(UPB_HANDLER_INT32);
@@ -545,6 +549,9 @@ static void visitfield(const upb_refcounted *r, upb_refcounted_visit *visit,
   if (upb_fielddef_containingtype(f)) {
     visit(r, UPB_UPCAST2(upb_fielddef_containingtype(f)), closure);
   }
+  if (upb_fielddef_containingoneof(f)) {
+    visit(r, UPB_UPCAST2(upb_fielddef_containingoneof(f)), closure);
+  }
   if (upb_fielddef_subdef(f)) {
     visit(r, UPB_UPCAST(upb_fielddef_subdef(f)), closure);
   }
@@ -620,6 +627,7 @@ upb_fielddef *upb_fielddef_new(const void *owner) {
   }
   f->msg.def = NULL;
   f->sub.def = NULL;
+  f->oneof = NULL;
   f->subdef_is_symbolic = false;
   f->msg_is_symbolic = false;
   f->label_ = UPB_LABEL_OPTIONAL;
@@ -749,6 +757,10 @@ const upb_msgdef *upb_fielddef_containingtype(const upb_fielddef *f) {
   return f->msg_is_symbolic ? NULL : f->msg.def;
 }
 
+const upb_oneofdef *upb_fielddef_containingoneof(const upb_fielddef *f) {
+  return f->oneof;
+}
+
 upb_msgdef *upb_fielddef_containingtype_mutable(upb_fielddef *f) {
   return (upb_msgdef*)upb_fielddef_containingtype(f);
 }
@@ -777,6 +789,10 @@ bool upb_fielddef_setcontainingtypename(upb_fielddef *f, const char *name,
 }
 
 bool upb_fielddef_setname(upb_fielddef *f, const char *name, upb_status *s) {
+  if (upb_fielddef_containingtype(f) || upb_fielddef_containingoneof(f)) {
+    upb_status_seterrmsg(s, "Already added to message or oneof");
+    return false;
+  }
   return upb_def_setfullname(UPB_UPCAST(f), name, s);
 }
 
@@ -1248,15 +1264,25 @@ bool upb_fielddef_checkdescriptortype(int32_t type) {
 static void visitmsg(const upb_refcounted *r, upb_refcounted_visit *visit,
                      void *closure) {
   const upb_msgdef *m = (const upb_msgdef*)r;
-  upb_msg_iter i;
-  for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, m);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     visit(r, UPB_UPCAST2(f), closure);
   }
+  upb_msg_oneof_iter o;
+  for(upb_msg_oneof_begin(&o, m);
+      !upb_msg_oneof_done(&o);
+      upb_msg_oneof_next(&o)) {
+    upb_oneofdef *f = upb_msg_iter_oneof(&o);
+    visit(r, UPB_UPCAST2(f), closure);
+  }
 }
 
 static void freemsg(upb_refcounted *r) {
   upb_msgdef *m = (upb_msgdef*)r;
+  upb_strtable_uninit(&m->ntoo);
   upb_strtable_uninit(&m->ntof);
   upb_inttable_uninit(&m->itof);
   upb_def_uninit(UPB_UPCAST(m));
@@ -1268,14 +1294,17 @@ upb_msgdef *upb_msgdef_new(const void *owner) {
   upb_msgdef *m = malloc(sizeof(*m));
   if (!m) return NULL;
   if (!upb_def_init(UPB_UPCAST(m), UPB_DEF_MSG, &vtbl, owner)) goto err2;
-  if (!upb_inttable_init(&m->itof, UPB_CTYPE_PTR)) goto err2;
-  if (!upb_strtable_init(&m->ntof, UPB_CTYPE_PTR)) goto err1;
+  if (!upb_inttable_init(&m->itof, UPB_CTYPE_PTR)) goto err3;
+  if (!upb_strtable_init(&m->ntof, UPB_CTYPE_PTR)) goto err2;
+  if (!upb_strtable_init(&m->ntoo, UPB_CTYPE_PTR)) goto err1;
   m->map_entry = false;
   return m;
 
 err1:
-  upb_inttable_uninit(&m->itof);
+  upb_strtable_uninit(&m->ntof);
 err2:
+  upb_inttable_uninit(&m->itof);
+err3:
   free(m);
   return NULL;
 }
@@ -1287,14 +1316,28 @@ upb_msgdef *upb_msgdef_dup(const upb_msgdef *m, const void *owner) {
                                 upb_def_fullname(UPB_UPCAST(m)), NULL);
   newm->map_entry = m->map_entry;
   UPB_ASSERT_VAR(ok, ok);
-  upb_msg_iter i;
-  for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, m);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_fielddef_dup(upb_msg_iter_field(&i), &f);
+    // Fields in oneofs are dup'd below.
+    if (upb_fielddef_containingoneof(f)) continue;
     if (!f || !upb_msgdef_addfield(newm, f, &f, NULL)) {
       upb_msgdef_unref(newm, owner);
       return NULL;
     }
   }
+  upb_msg_oneof_iter o;
+  for(upb_msg_oneof_begin(&o, m);
+      !upb_msg_oneof_done(&o);
+      upb_msg_oneof_next(&o)) {
+    upb_oneofdef *f = upb_oneofdef_dup(upb_msg_iter_oneof(&o), &f);
+    if (!f || !upb_msgdef_addoneof(newm, f, &f, NULL)) {
+      upb_msgdef_unref(newm, owner);
+      return NULL;
+    }
+  }
   return newm;
 }
 
@@ -1333,6 +1376,35 @@ bool upb_msgdef_setfullname(upb_msgdef *m, const char *fullname,
   return upb_def_setfullname(UPB_UPCAST(m), fullname, s);
 }
 
+// Helper: check that the field |f| is safe to add to msgdef |m|. Set an error
+// on status |s| and return false if not.
+static bool check_field_add(const upb_msgdef *m, const upb_fielddef *f,
+                            upb_status *s) {
+  if (upb_fielddef_containingtype(f) != NULL) {
+    upb_status_seterrmsg(s, "fielddef already belongs to a message");
+    return false;
+  } else if (upb_fielddef_name(f) == NULL || upb_fielddef_number(f) == 0) {
+    upb_status_seterrmsg(s, "field name or number were not set");
+    return false;
+  } else if (upb_msgdef_ntofz(m, upb_fielddef_name(f)) ||
+             upb_msgdef_itof(m, upb_fielddef_number(f))) {
+    upb_status_seterrmsg(s, "duplicate field name or number for field");
+    return false;
+  }
+  return true;
+}
+
+static void add_field(upb_msgdef *m, upb_fielddef *f, const void *ref_donor) {
+  release_containingtype(f);
+  f->msg.def = m;
+  f->msg_is_symbolic = false;
+  upb_inttable_insert(&m->itof, upb_fielddef_number(f), upb_value_ptr(f));
+  upb_strtable_insert(&m->ntof, upb_fielddef_name(f), upb_value_ptr(f));
+  upb_ref2(f, m);
+  upb_ref2(m, f);
+  if (ref_donor) upb_fielddef_unref(f, ref_donor);
+}
+
 bool upb_msgdef_addfield(upb_msgdef *m, upb_fielddef *f, const void *ref_donor,
                          upb_status *s) {
   // TODO: extensions need to have a separate namespace, because proto2 allows a
@@ -1346,28 +1418,65 @@ bool upb_msgdef_addfield(upb_msgdef *m, upb_fielddef *f, const void *ref_donor,
   // We also need to validate that the field number is in an extension range iff
   // it is an extension.
 
+  // This method is idempotent. Check if |f| is already part of this msgdef and
+  // return immediately if so.
+  if (upb_fielddef_containingtype(f) == m) {
+    return true;
+  }
+
   // Check constraints for all fields before performing any action.
-  if (upb_fielddef_containingtype(f) != NULL) {
-    upb_status_seterrmsg(s, "fielddef already belongs to a message");
+  if (!check_field_add(m, f, s)) {
     return false;
-  } else if (upb_fielddef_name(f) == NULL || upb_fielddef_number(f) == 0) {
-    upb_status_seterrmsg(s, "field name or number were not set");
-    return false;
-  } else if(upb_msgdef_itof(m, upb_fielddef_number(f)) ||
-            upb_msgdef_ntofz(m, upb_fielddef_name(f))) {
-    upb_status_seterrmsg(s, "duplicate field name or number");
+  } else if (upb_fielddef_containingoneof(f) != NULL) {
+    // Fields in a oneof can only be added by adding the oneof to the msgdef.
+    upb_status_seterrmsg(s, "fielddef is part of a oneof");
     return false;
   }
 
   // Constraint checks ok, perform the action.
-  release_containingtype(f);
-  f->msg.def = m;
-  f->msg_is_symbolic = false;
-  upb_inttable_insert(&m->itof, upb_fielddef_number(f), upb_value_ptr(f));
-  upb_strtable_insert(&m->ntof, upb_fielddef_name(f), upb_value_ptr(f));
-  upb_ref2(f, m);
-  upb_ref2(m, f);
-  if (ref_donor) upb_fielddef_unref(f, ref_donor);
+  add_field(m, f, ref_donor);
+  return true;
+}
+
+bool upb_msgdef_addoneof(upb_msgdef *m, upb_oneofdef *o, const void *ref_donor,
+                         upb_status *s) {
+  // Check various conditions that would prevent this oneof from being added.
+  if (upb_oneofdef_containingtype(o)) {
+    upb_status_seterrmsg(s, "oneofdef already belongs to a message");
+    return false;
+  } else if (upb_oneofdef_name(o) == NULL) {
+    upb_status_seterrmsg(s, "oneofdef name was not set");
+    return false;
+  } else if (upb_msgdef_ntooz(m, upb_oneofdef_name(o))) {
+    upb_status_seterrmsg(s, "duplicate oneof name");
+    return false;
+  }
+
+  // Check that all of the oneof's fields do not conflict with names or numbers
+  // of fields already in the message.
+  upb_oneof_iter it;
+  for (upb_oneof_begin(&it, o); !upb_oneof_done(&it); upb_oneof_next(&it)) {
+    const upb_fielddef *f = upb_oneof_iter_field(&it);
+    if (!check_field_add(m, f, s)) {
+      return false;
+    }
+  }
+
+  // Everything checks out -- commit now.
+
+  // Add oneof itself first.
+  o->parent = m;
+  upb_strtable_insert(&m->ntoo, upb_oneofdef_name(o), upb_value_ptr(o));
+  upb_ref2(o, m);
+  upb_ref2(m, o);
+
+  // Add each field of the oneof directly to the msgdef.
+  for (upb_oneof_begin(&it, o); !upb_oneof_done(&it); upb_oneof_next(&it)) {
+    upb_fielddef *f = upb_oneof_iter_field(&it);
+    add_field(m, f, NULL);
+  }
+
+  if (ref_donor) upb_oneofdef_unref(o, ref_donor);
 
   return true;
 }
@@ -1385,10 +1494,21 @@ const upb_fielddef *upb_msgdef_ntof(const upb_msgdef *m, const char *name,
       upb_value_getptr(val) : NULL;
 }
 
+const upb_oneofdef *upb_msgdef_ntoo(const upb_msgdef *m, const char *name,
+                                    size_t len) {
+  upb_value val;
+  return upb_strtable_lookup2(&m->ntoo, name, len, &val) ?
+      upb_value_getptr(val) : NULL;
+}
+
 int upb_msgdef_numfields(const upb_msgdef *m) {
   return upb_strtable_count(&m->ntof);
 }
 
+int upb_msgdef_numoneofs(const upb_msgdef *m) {
+  return upb_strtable_count(&m->ntoo);
+}
+
 void upb_msgdef_setmapentry(upb_msgdef *m, bool map_entry) {
   assert(!upb_msgdef_isfrozen(m));
   m->map_entry = map_entry;
@@ -1398,18 +1518,245 @@ bool upb_msgdef_mapentry(const upb_msgdef *m) {
   return m->map_entry;
 }
 
-void upb_msg_begin(upb_msg_iter *iter, const upb_msgdef *m) {
+void upb_msg_field_begin(upb_msg_field_iter *iter, const upb_msgdef *m) {
   upb_inttable_begin(iter, &m->itof);
 }
 
-void upb_msg_next(upb_msg_iter *iter) { upb_inttable_next(iter); }
+void upb_msg_field_next(upb_msg_field_iter *iter) { upb_inttable_next(iter); }
+
+bool upb_msg_field_done(const upb_msg_field_iter *iter) {
+  return upb_inttable_done(iter);
+}
+
+upb_fielddef *upb_msg_iter_field(const upb_msg_field_iter *iter) {
+  return (upb_fielddef*)upb_value_getptr(upb_inttable_iter_value(iter));
+}
+
+void upb_msg_field_iter_setdone(upb_msg_field_iter *iter) {
+  upb_inttable_iter_setdone(iter);
+}
+
+void upb_msg_oneof_begin(upb_msg_oneof_iter *iter, const upb_msgdef *m) {
+  upb_strtable_begin(iter, &m->ntoo);
+}
+
+void upb_msg_oneof_next(upb_msg_oneof_iter *iter) { upb_strtable_next(iter); }
+
+bool upb_msg_oneof_done(const upb_msg_oneof_iter *iter) {
+  return upb_strtable_done(iter);
+}
+
+upb_oneofdef *upb_msg_iter_oneof(const upb_msg_oneof_iter *iter) {
+  return (upb_oneofdef*)upb_value_getptr(upb_strtable_iter_value(iter));
+}
+
+void upb_msg_oneof_iter_setdone(upb_msg_oneof_iter *iter) {
+  upb_strtable_iter_setdone(iter);
+}
+
+/* upb_oneofdef ***************************************************************/
+
+static void visitoneof(const upb_refcounted *r, upb_refcounted_visit *visit,
+                       void *closure) {
+  const upb_oneofdef *o = (const upb_oneofdef*)r;
+  upb_oneof_iter i;
+  for (upb_oneof_begin(&i, o); !upb_oneof_done(&i); upb_oneof_next(&i)) {
+    const upb_fielddef *f = upb_oneof_iter_field(&i);
+    visit(r, UPB_UPCAST2(f), closure);
+  }
+  if (o->parent) {
+    visit(r, UPB_UPCAST2(o->parent), closure);
+  }
+}
+
+static void freeoneof(upb_refcounted *r) {
+  upb_oneofdef *o = (upb_oneofdef*)r;
+  upb_strtable_uninit(&o->ntof);
+  upb_inttable_uninit(&o->itof);
+  upb_def_uninit(UPB_UPCAST(o));
+  free(o);
+}
+
+upb_oneofdef *upb_oneofdef_new(const void *owner) {
+  static const struct upb_refcounted_vtbl vtbl = {visitoneof, freeoneof};
+  upb_oneofdef *o = malloc(sizeof(*o));
+  o->parent = NULL;
+  if (!o) return NULL;
+  if (!upb_def_init(UPB_UPCAST(o), UPB_DEF_ONEOF, &vtbl, owner)) goto err2;
+  if (!upb_inttable_init(&o->itof, UPB_CTYPE_PTR)) goto err2;
+  if (!upb_strtable_init(&o->ntof, UPB_CTYPE_PTR)) goto err1;
+  return o;
+
+err1:
+  upb_inttable_uninit(&o->itof);
+err2:
+  free(o);
+  return NULL;
+}
+
+upb_oneofdef *upb_oneofdef_dup(const upb_oneofdef *o, const void *owner) {
+  upb_oneofdef *newo = upb_oneofdef_new(owner);
+  if (!newo) return NULL;
+  bool ok = upb_def_setfullname(UPB_UPCAST(newo),
+                                upb_def_fullname(UPB_UPCAST(o)), NULL);
+  UPB_ASSERT_VAR(ok, ok);
+  upb_oneof_iter i;
+  for (upb_oneof_begin(&i, o); !upb_oneof_done(&i); upb_oneof_next(&i)) {
+    upb_fielddef *f = upb_fielddef_dup(upb_oneof_iter_field(&i), &f);
+    if (!f || !upb_oneofdef_addfield(newo, f, &f, NULL)) {
+      upb_oneofdef_unref(newo, owner);
+      return NULL;
+    }
+  }
+  return newo;
+}
+
+bool upb_oneofdef_isfrozen(const upb_oneofdef *o) {
+  return upb_def_isfrozen(UPB_UPCAST(o));
+}
+
+void upb_oneofdef_ref(const upb_oneofdef *o, const void *owner) {
+  upb_def_ref(UPB_UPCAST(o), owner);
+}
+
+void upb_oneofdef_unref(const upb_oneofdef *o, const void *owner) {
+  upb_def_unref(UPB_UPCAST(o), owner);
+}
+
+void upb_oneofdef_donateref(const upb_oneofdef *o, const void *from,
+                           const void *to) {
+  upb_def_donateref(UPB_UPCAST(o), from, to);
+}
+
+void upb_oneofdef_checkref(const upb_oneofdef *o, const void *owner) {
+  upb_def_checkref(UPB_UPCAST(o), owner);
+}
+
+const char *upb_oneofdef_name(const upb_oneofdef *o) {
+  return upb_def_fullname(UPB_UPCAST(o));
+}
+
+bool upb_oneofdef_setname(upb_oneofdef *o, const char *fullname,
+                             upb_status *s) {
+  if (upb_oneofdef_containingtype(o)) {
+    upb_status_seterrmsg(s, "oneof already added to a message");
+    return false;
+  }
+  return upb_def_setfullname(UPB_UPCAST(o), fullname, s);
+}
+
+const upb_msgdef *upb_oneofdef_containingtype(const upb_oneofdef *o) {
+  return o->parent;
+}
+
+int upb_oneofdef_numfields(const upb_oneofdef *o) {
+  return upb_strtable_count(&o->ntof);
+}
+
+bool upb_oneofdef_addfield(upb_oneofdef *o, upb_fielddef *f,
+                           const void *ref_donor,
+                           upb_status *s) {
+  assert(!upb_oneofdef_isfrozen(o));
+  assert(!o->parent || !upb_msgdef_isfrozen(o->parent));
 
-bool upb_msg_done(const upb_msg_iter *iter) { return upb_inttable_done(iter); }
+  // This method is idempotent. Check if |f| is already part of this oneofdef
+  // and return immediately if so.
+  if (upb_fielddef_containingoneof(f) == o) {
+    return true;
+  }
+
+  // The field must have an OPTIONAL label.
+  if (upb_fielddef_label(f) != UPB_LABEL_OPTIONAL) {
+    upb_status_seterrmsg(s, "fields in oneof must have OPTIONAL label");
+    return false;
+  }
+
+  // Check that no field with this name or number exists already in the oneof.
+  // Also check that the field is not already part of a oneof.
+  if (upb_fielddef_name(f) == NULL || upb_fielddef_number(f) == 0) {
+    upb_status_seterrmsg(s, "field name or number were not set");
+    return false;
+  } else if (upb_oneofdef_itof(o, upb_fielddef_number(f)) ||
+             upb_oneofdef_ntofz(o, upb_fielddef_name(f))) {
+    upb_status_seterrmsg(s, "duplicate field name or number");
+    return false;
+  } else if (upb_fielddef_containingoneof(f) != NULL) {
+    upb_status_seterrmsg(s, "fielddef already belongs to a oneof");
+    return false;
+  }
+
+  // We allow adding a field to the oneof either if the field is not part of a
+  // msgdef, or if it is and we are also part of the same msgdef.
+  if (o->parent == NULL) {
+    // If we're not in a msgdef, the field cannot be either. Otherwise we would
+    // need to magically add this oneof to a msgdef to remain consistent, which
+    // is surprising behavior.
+    if (upb_fielddef_containingtype(f) != NULL) {
+      upb_status_seterrmsg(s, "fielddef already belongs to a message, but "
+                              "oneof does not");
+      return false;
+    }
+  } else {
+    // If we're in a msgdef, the user can add fields that either aren't in any
+    // msgdef (in which case they're added to our msgdef) or already a part of
+    // our msgdef.
+    if (upb_fielddef_containingtype(f) != NULL &&
+        upb_fielddef_containingtype(f) != o->parent) {
+      upb_status_seterrmsg(s, "fielddef belongs to a different message "
+                              "than oneof");
+      return false;
+    }
+  }
+
+  // Commit phase. First add the field to our parent msgdef, if any, because
+  // that may fail; then add the field to our own tables.
+
+  if (o->parent != NULL && upb_fielddef_containingtype(f) == NULL) {
+    if (!upb_msgdef_addfield((upb_msgdef*)o->parent, f, NULL, s)) {
+      return false;
+    }
+  }
+
+  release_containingtype(f);
+  f->oneof = o;
+  upb_inttable_insert(&o->itof, upb_fielddef_number(f), upb_value_ptr(f));
+  upb_strtable_insert(&o->ntof, upb_fielddef_name(f), upb_value_ptr(f));
+  upb_ref2(f, o);
+  upb_ref2(o, f);
+  if (ref_donor) upb_fielddef_unref(f, ref_donor);
+
+  return true;
+}
+
+const upb_fielddef *upb_oneofdef_ntof(const upb_oneofdef *o,
+                                      const char *name, size_t length) {
+  upb_value val;
+  return upb_strtable_lookup2(&o->ntof, name, length, &val) ?
+      upb_value_getptr(val) : NULL;
+}
+
+const upb_fielddef *upb_oneofdef_itof(const upb_oneofdef *o, uint32_t num) {
+  upb_value val;
+  return upb_inttable_lookup32(&o->itof, num, &val) ?
+      upb_value_getptr(val) : NULL;
+}
+
+void upb_oneof_begin(upb_oneof_iter *iter, const upb_oneofdef *o) {
+  upb_inttable_begin(iter, &o->itof);
+}
+
+void upb_oneof_next(upb_oneof_iter *iter) {
+  upb_inttable_next(iter);
+}
+
+bool upb_oneof_done(upb_oneof_iter *iter) {
+  return upb_inttable_done(iter);
+}
 
-upb_fielddef *upb_msg_iter_field(const upb_msg_iter *iter) {
+upb_fielddef *upb_oneof_iter_field(const upb_oneof_iter *iter) {
   return (upb_fielddef*)upb_value_getptr(upb_inttable_iter_value(iter));
 }
 
-void upb_msg_iter_setdone(upb_msg_iter *iter) {
+void upb_oneof_iter_setdone(upb_oneof_iter *iter) {
   upb_inttable_iter_setdone(iter);
 }
diff --git a/upb/def.h b/upb/def.h
index ee138a05ec..fc342f3e28 100644
--- a/upb/def.h
+++ b/upb/def.h
@@ -34,6 +34,7 @@ class Def;
 class EnumDef;
 class FieldDef;
 class MessageDef;
+class OneofDef;
 }
 #endif
 
@@ -41,6 +42,7 @@ UPB_DECLARE_TYPE(upb::Def, upb_def);
 UPB_DECLARE_TYPE(upb::EnumDef, upb_enumdef);
 UPB_DECLARE_TYPE(upb::FieldDef, upb_fielddef);
 UPB_DECLARE_TYPE(upb::MessageDef, upb_msgdef);
+UPB_DECLARE_TYPE(upb::OneofDef, upb_oneofdef);
 
 // Maximum field number allowed for FieldDefs.  This is an inherent limit of the
 // protobuf wire format.
@@ -64,6 +66,7 @@ typedef enum {
   UPB_DEF_MSG,
   UPB_DEF_FIELD,
   UPB_DEF_ENUM,
+  UPB_DEF_ONEOF,
   UPB_DEF_SERVICE,   // Not yet implemented.
   UPB_DEF_ANY = -1,  // Wildcard for upb_symtab_get*()
 } upb_deftype_t;
@@ -348,6 +351,10 @@ UPB_DEFINE_DEF(upb::FieldDef, fielddef, FIELD,
   const MessageDef* containing_type() const;
   const char* containing_type_name();
 
+  // The OneofDef to which this field belongs, or NULL if this field is not part
+  // of a oneof.
+  const OneofDef* containing_oneof() const;
+
   // The field's type according to the enum in descriptor.proto.  This is not
   // the same as UPB_TYPE_*, because it distinguishes between (for example)
   // INT32 and SINT32, whereas our "type" enum does not.  This return of
@@ -521,6 +528,7 @@ UPB_DEFINE_STRUCT(upb_fielddef, upb_def,
   } sub;  // The msgdef or enumdef for this field, if upb_hassubdef(f).
   bool subdef_is_symbolic;
   bool msg_is_symbolic;
+  const upb_oneofdef *oneof;
   bool default_is_string;
   bool type_is_set_;     // False until type is explicitly set.
   bool is_extension_;
@@ -536,11 +544,11 @@ UPB_DEFINE_STRUCT(upb_fielddef, upb_def,
 ));
 
 #define UPB_FIELDDEF_INIT(label, type, intfmt, tagdelim, is_extension, lazy,   \
-                          packed, name, num, msgdef, subdef, selector_base,     \
+                          packed, name, num, msgdef, subdef, selector_base,    \
                           index, defaultval, refs, ref2s)                      \
   {                                                                            \
     UPB_DEF_INIT(name, UPB_DEF_FIELD, refs, ref2s), defaultval, {msgdef},      \
-        {subdef}, false, false,                                                \
+        {subdef}, NULL, false, false,                                          \
         type == UPB_TYPE_STRING || type == UPB_TYPE_BYTES, true, is_extension, \
         lazy, packed, intfmt, tagdelim, type, label, num, selector_base, index \
   }
@@ -574,6 +582,7 @@ bool upb_fielddef_isextension(const upb_fielddef *f);
 bool upb_fielddef_lazy(const upb_fielddef *f);
 bool upb_fielddef_packed(const upb_fielddef *f);
 const upb_msgdef *upb_fielddef_containingtype(const upb_fielddef *f);
+const upb_oneofdef *upb_fielddef_containingoneof(const upb_fielddef *f);
 upb_msgdef *upb_fielddef_containingtype_mutable(upb_fielddef *f);
 const char *upb_fielddef_containingtypename(upb_fielddef *f);
 upb_intfmt_t upb_fielddef_intfmt(const upb_fielddef *f);
@@ -641,7 +650,8 @@ UPB_END_EXTERN_C  // }
 
 /* upb::MessageDef ************************************************************/
 
-typedef upb_inttable_iter upb_msg_iter;
+typedef upb_inttable_iter upb_msg_field_iter;
+typedef upb_strtable_iter upb_msg_oneof_iter;
 
 // Structure that describes a single .proto message type.
 //
@@ -671,14 +681,37 @@ UPB_DEFINE_DEF(upb::MessageDef, msgdef, MSG, UPB_QUOTE(
   // The number of fields that belong to the MessageDef.
   int field_count() const;
 
+  // The number of oneofs that belong to the MessageDef.
+  int oneof_count() const;
+
   // Adds a field (upb_fielddef object) to a msgdef.  Requires that the msgdef
   // and the fielddefs are mutable.  The fielddef's name and number must be
   // set, and the message may not already contain any field with this name or
   // number, and this fielddef may not be part of another message.  In error
   // cases false is returned and the msgdef is unchanged.
+  //
+  // If the given field is part of a oneof, this call succeeds if and only if
+  // that oneof is already part of this msgdef. (Note that adding a oneof to a
+  // msgdef automatically adds all of its fields to the msgdef at the time that
+  // the oneof is added, so it is usually more idiomatic to add the oneof's
+  // fields first then add the oneof to the msgdef. This case is supported for
+  // convenience.)
+  //
+  // If |f| is already part of this MessageDef, this method performs no action
+  // and returns true (success). Thus, this method is idempotent.
   bool AddField(FieldDef* f, Status* s);
   bool AddField(const reffed_ptr<FieldDef>& f, Status* s);
 
+  // Adds a oneof (upb_oneofdef object) to a msgdef. Requires that the msgdef,
+  // oneof, and any fielddefs are mutable, that the fielddefs contained in the
+  // oneof do not have any name or number conflicts with existing fields in the
+  // msgdef, and that the oneof's name is unique among all oneofs in the msgdef.
+  // If the oneof is added successfully, all of its fields will be added
+  // directly to the msgdef as well. In error cases, false is returned and the
+  // msgdef is unchanged.
+  bool AddOneof(OneofDef* o, Status* s);
+  bool AddOneof(const reffed_ptr<OneofDef>& o, Status* s);
+
   // These return NULL if the field is not found.
   FieldDef* FindFieldByNumber(uint32_t number);
   FieldDef* FindFieldByName(const char *name, size_t len);
@@ -702,6 +735,25 @@ UPB_DEFINE_DEF(upb::MessageDef, msgdef, MSG, UPB_QUOTE(
     return FindFieldByName(str.c_str(), str.size());
   }
 
+  OneofDef* FindOneofByName(const char* name, size_t len);
+  const OneofDef* FindOneofByName(const char* name, size_t len) const;
+
+  OneofDef* FindOneofByName(const char* name) {
+    return FindOneofByName(name, strlen(name));
+  }
+  const OneofDef* FindOneofByName(const char* name) const {
+    return FindOneofByName(name, strlen(name));
+  }
+
+  template<class T>
+  OneofDef* FindOneofByName(const T& str) {
+    return FindOneofByName(str.c_str(), str.size());
+  }
+  template<class T>
+  const OneofDef* FindOneofByName(const T& str) const {
+    return FindOneofByName(str.c_str(), str.size());
+  }
+
   // Returns a new msgdef that is a copy of the given msgdef (and a copy of all
   // the fields) but with any references to submessages broken and replaced
   // with just the name of the submessage.  Returns NULL if memory allocation
@@ -717,39 +769,117 @@ UPB_DEFINE_DEF(upb::MessageDef, msgdef, MSG, UPB_QUOTE(
   bool mapentry() const;
 
   // Iteration over fields.  The order is undefined.
-  class iterator : public std::iterator<std::forward_iterator_tag, FieldDef*> {
+  class field_iterator
+      : public std::iterator<std::forward_iterator_tag, FieldDef*> {
    public:
-    explicit iterator(MessageDef* md);
-    static iterator end(MessageDef* md);
+    explicit field_iterator(MessageDef* md);
+    static field_iterator end(MessageDef* md);
 
     void operator++();
     FieldDef* operator*() const;
-    bool operator!=(const iterator& other) const;
-    bool operator==(const iterator& other) const;
+    bool operator!=(const field_iterator& other) const;
+    bool operator==(const field_iterator& other) const;
 
    private:
-    upb_msg_iter iter_;
+    upb_msg_field_iter iter_;
   };
 
-  class const_iterator
+  class const_field_iterator
       : public std::iterator<std::forward_iterator_tag, const FieldDef*> {
    public:
-    explicit const_iterator(const MessageDef* md);
-    static const_iterator end(const MessageDef* md);
+    explicit const_field_iterator(const MessageDef* md);
+    static const_field_iterator end(const MessageDef* md);
 
     void operator++();
     const FieldDef* operator*() const;
-    bool operator!=(const const_iterator& other) const;
-    bool operator==(const const_iterator& other) const;
+    bool operator!=(const const_field_iterator& other) const;
+    bool operator==(const const_field_iterator& other) const;
 
    private:
-    upb_msg_iter iter_;
+    upb_msg_field_iter iter_;
   };
 
-  iterator begin();
-  iterator end();
-  const_iterator begin() const;
-  const_iterator end() const;
+  // Iteration over oneofs. The order is undefined.
+  class oneof_iterator
+      : public std::iterator<std::forward_iterator_tag, FieldDef*> {
+   public:
+    explicit oneof_iterator(MessageDef* md);
+    static oneof_iterator end(MessageDef* md);
+
+    void operator++();
+    OneofDef* operator*() const;
+    bool operator!=(const oneof_iterator& other) const;
+    bool operator==(const oneof_iterator& other) const;
+
+   private:
+    upb_msg_oneof_iter iter_;
+  };
+
+  class const_oneof_iterator
+      : public std::iterator<std::forward_iterator_tag, const FieldDef*> {
+   public:
+    explicit const_oneof_iterator(const MessageDef* md);
+    static const_oneof_iterator end(const MessageDef* md);
+
+    void operator++();
+    const OneofDef* operator*() const;
+    bool operator!=(const const_oneof_iterator& other) const;
+    bool operator==(const const_oneof_iterator& other) const;
+
+   private:
+    upb_msg_oneof_iter iter_;
+  };
+
+  class FieldAccessor {
+   public:
+    explicit FieldAccessor(MessageDef* msg) : msg_(msg) {}
+    field_iterator begin() { return msg_->field_begin(); }
+    field_iterator end() { return msg_->field_end(); }
+   private:
+    MessageDef* msg_;
+  };
+
+  class ConstFieldAccessor {
+   public:
+    explicit ConstFieldAccessor(const MessageDef* msg) : msg_(msg) {}
+    const_field_iterator begin() { return msg_->field_begin(); }
+    const_field_iterator end() { return msg_->field_end(); }
+   private:
+    const MessageDef* msg_;
+  };
+
+  class OneofAccessor {
+   public:
+    explicit OneofAccessor(MessageDef* msg) : msg_(msg) {}
+    oneof_iterator begin() { return msg_->oneof_begin(); }
+    oneof_iterator end() { return msg_->oneof_end(); }
+   private:
+    MessageDef* msg_;
+  };
+
+  class ConstOneofAccessor {
+   public:
+    explicit ConstOneofAccessor(const MessageDef* msg) : msg_(msg) {}
+    const_oneof_iterator begin() { return msg_->oneof_begin(); }
+    const_oneof_iterator end() { return msg_->oneof_end(); }
+   private:
+    const MessageDef* msg_;
+  };
+
+  field_iterator field_begin();
+  field_iterator field_end();
+  const_field_iterator field_begin() const;
+  const_field_iterator field_end() const;
+
+  oneof_iterator oneof_begin();
+  oneof_iterator oneof_end();
+  const_oneof_iterator oneof_begin() const;
+  const_oneof_iterator oneof_end() const;
+
+  FieldAccessor fields() { return FieldAccessor(this); }
+  ConstFieldAccessor fields() const { return ConstFieldAccessor(this); }
+  OneofAccessor oneofs() { return OneofAccessor(this); }
+  ConstOneofAccessor oneofs() const { return ConstOneofAccessor(this); }
 
  private:
   UPB_DISALLOW_POD_OPS(MessageDef, upb::MessageDef);
@@ -762,6 +892,9 @@ UPB_DEFINE_STRUCT(upb_msgdef, upb_def,
   upb_inttable itof;  // int to field
   upb_strtable ntof;  // name to field
 
+  // Tables for looking up oneofs by name.
+  upb_strtable ntoo;  // name to oneof
+
   // Is this a map-entry message?
   // TODO: set this flag properly for static descriptors; regenerate
   // descriptor.upb.c.
@@ -770,11 +903,14 @@ UPB_DEFINE_STRUCT(upb_msgdef, upb_def,
   // TODO(haberman): proper extension ranges (there can be multiple).
 ));
 
+// TODO: also support static initialization of the oneofs table. This will be
+// needed if we compile in descriptors that contain oneofs.
 #define UPB_MSGDEF_INIT(name, selector_count, submsg_field_count, itof, ntof, \
                         refs, ref2s)                                          \
   {                                                                           \
     UPB_DEF_INIT(name, UPB_DEF_MSG, refs, ref2s), selector_count,             \
-        submsg_field_count, itof, ntof, false                                 \
+        submsg_field_count, itof, ntof,                                       \
+        UPB_EMPTY_STRTABLE_INIT(UPB_CTYPE_PTR), false                         \
   }
 
 UPB_BEGIN_EXTERN_C  // {
@@ -798,6 +934,8 @@ bool upb_msgdef_setfullname(upb_msgdef *m, const char *fullname, upb_status *s);
 upb_msgdef *upb_msgdef_dup(const upb_msgdef *m, const void *owner);
 bool upb_msgdef_addfield(upb_msgdef *m, upb_fielddef *f, const void *ref_donor,
                          upb_status *s);
+bool upb_msgdef_addoneof(upb_msgdef *m, upb_oneofdef *o, const void *ref_donor,
+                         upb_status *s);
 
 // Field lookup in a couple of different variations:
 //   - itof = int to field
@@ -822,11 +960,34 @@ UPB_INLINE upb_fielddef *upb_msgdef_ntof_mutable(upb_msgdef *m,
   return (upb_fielddef *)upb_msgdef_ntof(m, name, len);
 }
 
+// Oneof lookup:
+//   - ntoo = name to oneof
+//   - ntooz = name to oneof, null-terminated string.
+const upb_oneofdef *upb_msgdef_ntoo(const upb_msgdef *m, const char *name,
+                                    size_t len);
+int upb_msgdef_numoneofs(const upb_msgdef *m);
+
+UPB_INLINE const upb_oneofdef *upb_msgdef_ntooz(const upb_msgdef *m,
+                                               const char *name) {
+  return upb_msgdef_ntoo(m, name, strlen(name));
+}
+
+UPB_INLINE upb_oneofdef *upb_msgdef_ntoo_mutable(upb_msgdef *m,
+                                                 const char *name, size_t len) {
+  return (upb_oneofdef *)upb_msgdef_ntoo(m, name, len);
+}
+
 void upb_msgdef_setmapentry(upb_msgdef *m, bool map_entry);
 bool upb_msgdef_mapentry(const upb_msgdef *m);
 
-// upb_msg_iter i;
-// for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
+const upb_oneofdef *upb_msgdef_findoneof(const upb_msgdef *m,
+                                          const char *name);
+int upb_msgdef_numoneofs(const upb_msgdef *m);
+
+// upb_msg_field_iter i;
+// for(upb_msg_field_begin(&i, m);
+//     !upb_msg_field_done(&i);
+//     upb_msg_field_next(&i)) {
 //   upb_fielddef *f = upb_msg_iter_field(&i);
 //   // ...
 // }
@@ -834,11 +995,18 @@ bool upb_msgdef_mapentry(const upb_msgdef *m);
 // For C we don't have separate iterators for const and non-const.
 // It is the caller's responsibility to cast the upb_fielddef* to
 // const if the upb_msgdef* is const.
-void upb_msg_begin(upb_msg_iter *iter, const upb_msgdef *m);
-void upb_msg_next(upb_msg_iter *iter);
-bool upb_msg_done(const upb_msg_iter *iter);
-upb_fielddef *upb_msg_iter_field(const upb_msg_iter *iter);
-void upb_msg_iter_setdone(upb_msg_iter *iter);
+void upb_msg_field_begin(upb_msg_field_iter *iter, const upb_msgdef *m);
+void upb_msg_field_next(upb_msg_field_iter *iter);
+bool upb_msg_field_done(const upb_msg_field_iter *iter);
+upb_fielddef *upb_msg_iter_field(const upb_msg_field_iter *iter);
+void upb_msg_field_iter_setdone(upb_msg_field_iter *iter);
+
+// Similar to above, we also support iterating through the oneofs in a msgdef.
+void upb_msg_oneof_begin(upb_msg_oneof_iter *iter, const upb_msgdef *m);
+void upb_msg_oneof_next(upb_msg_oneof_iter *iter);
+bool upb_msg_oneof_done(const upb_msg_oneof_iter *iter);
+upb_oneofdef *upb_msg_iter_oneof(const upb_msg_oneof_iter *iter);
+void upb_msg_oneof_iter_setdone(upb_msg_oneof_iter *iter);
 
 UPB_END_EXTERN_C  // }
 
@@ -980,6 +1148,172 @@ int32_t upb_enum_iter_number(upb_enum_iter *iter);
 
 UPB_END_EXTERN_C  // }
 
+/* upb::OneofDef **************************************************************/
+
+typedef upb_inttable_iter upb_oneof_iter;
+
+// Class that represents a oneof.  Its base class is upb::Def (convert with
+// upb::upcast()).
+UPB_DEFINE_DEF(upb::OneofDef, oneofdef, ONEOF, UPB_QUOTE(
+ public:
+  // Returns NULL if memory allocation failed.
+  static reffed_ptr<OneofDef> New();
+
+  // Functionality from upb::RefCounted.
+  bool IsFrozen() const;
+  void Ref(const void* owner) const;
+  void Unref(const void* owner) const;
+  void DonateRef(const void* from, const void* to) const;
+  void CheckRef(const void* owner) const;
+
+  // Functionality from upb::Def.
+  const char* full_name() const;
+
+  // Returns the MessageDef that owns this OneofDef.
+  const MessageDef* containing_type() const;
+
+  // Returns the name of this oneof. This is the name used to look up the oneof
+  // by name once added to a message def.
+  const char* name() const;
+  bool set_name(const char* name, Status* s);
+
+  // Returns the number of fields currently defined in the oneof.
+  int field_count() const;
+
+  // Adds a field to the oneof. The field must not have been added to any other
+  // oneof or msgdef. If the oneof is not yet part of a msgdef, then when the
+  // oneof is eventually added to a msgdef, all fields added to the oneof will
+  // also be added to the msgdef at that time. If the oneof is already part of a
+  // msgdef, the field must either be a part of that msgdef already, or must not
+  // be a part of any msgdef; in the latter case, the field is added to the
+  // msgdef as a part of this operation.
+  //
+  // The field may only have an OPTIONAL label, never REQUIRED or REPEATED.
+  //
+  // If |f| is already part of this MessageDef, this method performs no action
+  // and returns true (success). Thus, this method is idempotent.
+  bool AddField(FieldDef* field, Status* s);
+  bool AddField(const reffed_ptr<FieldDef>& field, Status* s);
+
+  // Looks up by name.
+  const FieldDef* FindFieldByName(const char* name, size_t len) const;
+  FieldDef* FindFieldByName(const char* name, size_t len);
+  const FieldDef* FindFieldByName(const char* name) const {
+    return FindFieldByName(name, strlen(name));
+  }
+  FieldDef* FindFieldByName(const char* name) {
+    return FindFieldByName(name, strlen(name));
+  }
+
+  template <class T>
+  FieldDef* FindFieldByName(const T& str) {
+    return FindFieldByName(str.c_str(), str.size());
+  }
+  template <class T>
+  const FieldDef* FindFieldByName(const T& str) const {
+    return FindFieldByName(str.c_str(), str.size());
+  }
+
+  // Looks up by tag number.
+  const FieldDef* FindFieldByNumber(uint32_t num) const;
+
+  // Returns a new OneofDef with all the same fields. The OneofDef will be owned
+  // by the given owner.
+  OneofDef* Dup(const void* owner) const;
+
+  // Iteration over fields.  The order is undefined.
+  class iterator : public std::iterator<std::forward_iterator_tag, FieldDef*> {
+   public:
+    explicit iterator(OneofDef* md);
+    static iterator end(OneofDef* md);
+
+    void operator++();
+    FieldDef* operator*() const;
+    bool operator!=(const iterator& other) const;
+    bool operator==(const iterator& other) const;
+
+   private:
+    upb_oneof_iter iter_;
+  };
+
+  class const_iterator
+      : public std::iterator<std::forward_iterator_tag, const FieldDef*> {
+   public:
+    explicit const_iterator(const OneofDef* md);
+    static const_iterator end(const OneofDef* md);
+
+    void operator++();
+    const FieldDef* operator*() const;
+    bool operator!=(const const_iterator& other) const;
+    bool operator==(const const_iterator& other) const;
+
+   private:
+    upb_oneof_iter iter_;
+  };
+
+  iterator begin();
+  iterator end();
+  const_iterator begin() const;
+  const_iterator end() const;
+
+ private:
+  UPB_DISALLOW_POD_OPS(OneofDef, upb::OneofDef);
+),
+UPB_DEFINE_STRUCT(upb_oneofdef, upb_def,
+  upb_strtable ntof;
+  upb_inttable itof;
+  const upb_msgdef *parent;
+));
+
+#define UPB_ONEOFDEF_INIT(name, ntof, itof, refs, ref2s) \
+  { UPB_DEF_INIT(name, UPB_DEF_ENUM, refs, ref2s), ntof, itof }
+
+UPB_BEGIN_EXTERN_C  // {
+
+// Native C API.
+upb_oneofdef *upb_oneofdef_new(const void *owner);
+upb_oneofdef *upb_oneofdef_dup(const upb_oneofdef *o, const void *owner);
+
+// From upb_refcounted.
+void upb_oneofdef_unref(const upb_oneofdef *o, const void *owner);
+bool upb_oneofdef_isfrozen(const upb_oneofdef *e);
+void upb_oneofdef_ref(const upb_oneofdef *o, const void *owner);
+void upb_oneofdef_donateref(const upb_oneofdef *m, const void *from,
+                           const void *to);
+void upb_oneofdef_checkref(const upb_oneofdef *o, const void *owner);
+
+const char *upb_oneofdef_name(const upb_oneofdef *o);
+bool upb_oneofdef_setname(upb_oneofdef *o, const char *name, upb_status *s);
+
+const upb_msgdef *upb_oneofdef_containingtype(const upb_oneofdef *o);
+int upb_oneofdef_numfields(const upb_oneofdef *o);
+bool upb_oneofdef_addfield(upb_oneofdef *o, upb_fielddef *f,
+                           const void *ref_donor,
+                           upb_status *s);
+
+// Oneof lookups:
+// - ntof:  look up a field by name.
+// - ntofz: look up a field by name (as a null-terminated string).
+// - itof:  look up a field by number.
+const upb_fielddef *upb_oneofdef_ntof(const upb_oneofdef *o,
+                                      const char *name, size_t length);
+UPB_INLINE const upb_fielddef *upb_oneofdef_ntofz(const upb_oneofdef *o,
+                                                  const char *name) {
+  return upb_oneofdef_ntof(o, name, strlen(name));
+}
+const upb_fielddef *upb_oneofdef_itof(const upb_oneofdef *o, uint32_t num);
+
+//  upb_oneof_iter i;
+//  for(upb_oneof_begin(&i, e); !upb_oneof_done(&i); upb_oneof_next(&i)) {
+//    // ...
+//  }
+void upb_oneof_begin(upb_oneof_iter *iter, const upb_oneofdef *o);
+void upb_oneof_next(upb_oneof_iter *iter);
+bool upb_oneof_done(upb_oneof_iter *iter);
+upb_fielddef *upb_oneof_iter_field(const upb_oneof_iter *iter);
+void upb_oneof_iter_setdone(upb_oneof_iter *iter);
+
+UPB_END_EXTERN_C  // }
 
 #ifdef __cplusplus
 
@@ -1106,6 +1440,9 @@ inline void FieldDef::set_packed(bool packed) {
 inline const MessageDef* FieldDef::containing_type() const {
   return upb_fielddef_containingtype(this);
 }
+inline const OneofDef* FieldDef::containing_oneof() const {
+  return upb_fielddef_containingoneof(this);
+}
 inline const char* FieldDef::containing_type_name() {
   return upb_fielddef_containingtypename(this);
 }
@@ -1256,12 +1593,21 @@ inline bool MessageDef::Freeze(Status* status) {
 inline int MessageDef::field_count() const {
   return upb_msgdef_numfields(this);
 }
+inline int MessageDef::oneof_count() const {
+  return upb_msgdef_numoneofs(this);
+}
 inline bool MessageDef::AddField(upb_fielddef* f, Status* s) {
   return upb_msgdef_addfield(this, f, NULL, s);
 }
 inline bool MessageDef::AddField(const reffed_ptr<FieldDef>& f, Status* s) {
   return upb_msgdef_addfield(this, f.get(), NULL, s);
 }
+inline bool MessageDef::AddOneof(upb_oneofdef* o, Status* s) {
+  return upb_msgdef_addoneof(this, o, NULL, s);
+}
+inline bool MessageDef::AddOneof(const reffed_ptr<OneofDef>& o, Status* s) {
+  return upb_msgdef_addoneof(this, o.get(), NULL, s);
+}
 inline FieldDef* MessageDef::FindFieldByNumber(uint32_t number) {
   return upb_msgdef_itof_mutable(this, number);
 }
@@ -1275,6 +1621,13 @@ inline const FieldDef *MessageDef::FindFieldByName(const char *name,
                                                    size_t len) const {
   return upb_msgdef_ntof(this, name, len);
 }
+inline OneofDef* MessageDef::FindOneofByName(const char* name, size_t len) {
+  return upb_msgdef_ntoo_mutable(this, name, len);
+}
+inline const OneofDef* MessageDef::FindOneofByName(const char* name,
+                                                   size_t len) const {
+  return upb_msgdef_ntoo(this, name, len);
+}
 inline MessageDef* MessageDef::Dup(const void *owner) const {
   return upb_msgdef_dup(this, owner);
 }
@@ -1284,55 +1637,127 @@ inline void MessageDef::setmapentry(bool map_entry) {
 inline bool MessageDef::mapentry() const {
   return upb_msgdef_mapentry(this);
 }
-inline MessageDef::iterator MessageDef::begin() { return iterator(this); }
-inline MessageDef::iterator MessageDef::end() { return iterator::end(this); }
-inline MessageDef::const_iterator MessageDef::begin() const {
-  return const_iterator(this);
+inline MessageDef::field_iterator MessageDef::field_begin() {
+  return field_iterator(this);
 }
-inline MessageDef::const_iterator MessageDef::end() const {
-  return const_iterator::end(this);
+inline MessageDef::field_iterator MessageDef::field_end() {
+  return field_iterator::end(this);
+}
+inline MessageDef::const_field_iterator MessageDef::field_begin() const {
+  return const_field_iterator(this);
+}
+inline MessageDef::const_field_iterator MessageDef::field_end() const {
+  return const_field_iterator::end(this);
 }
 
-inline MessageDef::iterator::iterator(MessageDef* md) {
-  upb_msg_begin(&iter_, md);
+inline MessageDef::oneof_iterator MessageDef::oneof_begin() {
+  return oneof_iterator(this);
+}
+inline MessageDef::oneof_iterator MessageDef::oneof_end() {
+  return oneof_iterator::end(this);
+}
+inline MessageDef::const_oneof_iterator MessageDef::oneof_begin() const {
+  return const_oneof_iterator(this);
+}
+inline MessageDef::const_oneof_iterator MessageDef::oneof_end() const {
+  return const_oneof_iterator::end(this);
 }
-inline MessageDef::iterator MessageDef::iterator::end(MessageDef* md) {
-  MessageDef::iterator iter(md);
-  upb_msg_iter_setdone(&iter.iter_);
+
+inline MessageDef::field_iterator::field_iterator(MessageDef* md) {
+  upb_msg_field_begin(&iter_, md);
+}
+inline MessageDef::field_iterator MessageDef::field_iterator::end(
+    MessageDef* md) {
+  MessageDef::field_iterator iter(md);
+  upb_msg_field_iter_setdone(&iter.iter_);
   return iter;
 }
-inline FieldDef* MessageDef::iterator::operator*() const {
+inline FieldDef* MessageDef::field_iterator::operator*() const {
   return upb_msg_iter_field(&iter_);
 }
-inline void MessageDef::iterator::operator++() { return upb_msg_next(&iter_); }
-inline bool MessageDef::iterator::operator==(const iterator &other) const {
+inline void MessageDef::field_iterator::operator++() {
+  return upb_msg_field_next(&iter_);
+}
+inline bool MessageDef::field_iterator::operator==(
+    const field_iterator &other) const {
   return upb_inttable_iter_isequal(&iter_, &other.iter_);
 }
-inline bool MessageDef::iterator::operator!=(const iterator &other) const {
+inline bool MessageDef::field_iterator::operator!=(
+    const field_iterator &other) const {
   return !(*this == other);
 }
 
-inline MessageDef::const_iterator::const_iterator(const MessageDef* md) {
-  upb_msg_begin(&iter_, md);
+inline MessageDef::const_field_iterator::const_field_iterator(
+    const MessageDef* md) {
+  upb_msg_field_begin(&iter_, md);
 }
-inline MessageDef::const_iterator MessageDef::const_iterator::end(
+inline MessageDef::const_field_iterator MessageDef::const_field_iterator::end(
     const MessageDef *md) {
-  MessageDef::const_iterator iter(md);
-  upb_msg_iter_setdone(&iter.iter_);
+  MessageDef::const_field_iterator iter(md);
+  upb_msg_field_iter_setdone(&iter.iter_);
   return iter;
 }
-inline const FieldDef* MessageDef::const_iterator::operator*() const {
+inline const FieldDef* MessageDef::const_field_iterator::operator*() const {
   return upb_msg_iter_field(&iter_);
 }
-inline void MessageDef::const_iterator::operator++() {
-  return upb_msg_next(&iter_);
+inline void MessageDef::const_field_iterator::operator++() {
+  return upb_msg_field_next(&iter_);
 }
-inline bool MessageDef::const_iterator::operator==(
-    const const_iterator &other) const {
+inline bool MessageDef::const_field_iterator::operator==(
+    const const_field_iterator &other) const {
   return upb_inttable_iter_isequal(&iter_, &other.iter_);
 }
-inline bool MessageDef::const_iterator::operator!=(
-    const const_iterator &other) const {
+inline bool MessageDef::const_field_iterator::operator!=(
+    const const_field_iterator &other) const {
+  return !(*this == other);
+}
+
+inline MessageDef::oneof_iterator::oneof_iterator(MessageDef* md) {
+  upb_msg_oneof_begin(&iter_, md);
+}
+inline MessageDef::oneof_iterator MessageDef::oneof_iterator::end(
+    MessageDef* md) {
+  MessageDef::oneof_iterator iter(md);
+  upb_msg_oneof_iter_setdone(&iter.iter_);
+  return iter;
+}
+inline OneofDef* MessageDef::oneof_iterator::operator*() const {
+  return upb_msg_iter_oneof(&iter_);
+}
+inline void MessageDef::oneof_iterator::operator++() {
+  return upb_msg_oneof_next(&iter_);
+}
+inline bool MessageDef::oneof_iterator::operator==(
+    const oneof_iterator &other) const {
+  return upb_strtable_iter_isequal(&iter_, &other.iter_);
+}
+inline bool MessageDef::oneof_iterator::operator!=(
+    const oneof_iterator &other) const {
+  return !(*this == other);
+}
+
+inline MessageDef::const_oneof_iterator::const_oneof_iterator(
+    const MessageDef* md) {
+  upb_msg_oneof_begin(&iter_, md);
+}
+inline MessageDef::const_oneof_iterator MessageDef::const_oneof_iterator::end(
+    const MessageDef *md) {
+  MessageDef::const_oneof_iterator iter(md);
+  upb_msg_oneof_iter_setdone(&iter.iter_);
+  return iter;
+}
+inline const OneofDef* MessageDef::const_oneof_iterator::operator*() const {
+  return upb_msg_iter_oneof(&iter_);
+}
+inline void MessageDef::const_oneof_iterator::operator++() {
+  return upb_msg_oneof_next(&iter_);
+}
+inline bool MessageDef::const_oneof_iterator::operator==(
+    const const_oneof_iterator &other) const {
+  return upb_strtable_iter_isequal(&iter_, &other.iter_);
+}
+inline bool MessageDef::const_oneof_iterator::operator!=(
+    const const_oneof_iterator &other) const {
   return !(*this == other);
 }
 
@@ -1400,6 +1825,105 @@ inline const char* EnumDef::Iterator::name() {
 }
 inline bool EnumDef::Iterator::Done() { return upb_enum_done(&iter_); }
 inline void EnumDef::Iterator::Next() { return upb_enum_next(&iter_); }
+
+inline reffed_ptr<OneofDef> OneofDef::New() {
+  upb_oneofdef *o = upb_oneofdef_new(&o);
+  return reffed_ptr<OneofDef>(o, &o);
+}
+inline bool OneofDef::IsFrozen() const { return upb_oneofdef_isfrozen(this); }
+inline void OneofDef::Ref(const void* owner) const {
+  return upb_oneofdef_ref(this, owner);
+}
+inline void OneofDef::Unref(const void* owner) const {
+  return upb_oneofdef_unref(this, owner);
+}
+inline void OneofDef::DonateRef(const void* from, const void* to) const {
+  return upb_oneofdef_donateref(this, from, to);
+}
+inline void OneofDef::CheckRef(const void* owner) const {
+  return upb_oneofdef_checkref(this, owner);
+}
+inline const char* OneofDef::full_name() const {
+  return upb_oneofdef_name(this);
+}
+
+inline const MessageDef* OneofDef::containing_type() const {
+  return upb_oneofdef_containingtype(this);
+}
+inline const char* OneofDef::name() const {
+  return upb_oneofdef_name(this);
+}
+inline bool OneofDef::set_name(const char* name, Status* s) {
+  return upb_oneofdef_setname(this, name, s);
+}
+inline int OneofDef::field_count() const {
+  return upb_oneofdef_numfields(this);
+}
+inline bool OneofDef::AddField(FieldDef* field, Status* s) {
+  return upb_oneofdef_addfield(this, field, NULL, s);
+}
+inline bool OneofDef::AddField(const reffed_ptr<FieldDef>& field, Status* s) {
+  return upb_oneofdef_addfield(this, field.get(), NULL, s);
+}
+inline const FieldDef* OneofDef::FindFieldByName(const char* name,
+                                                 size_t len) const {
+  return upb_oneofdef_ntof(this, name, len);
+}
+inline const FieldDef* OneofDef::FindFieldByNumber(uint32_t num) const {
+  return upb_oneofdef_itof(this, num);
+}
+inline OneofDef::iterator OneofDef::begin() { return iterator(this); }
+inline OneofDef::iterator OneofDef::end() { return iterator::end(this); }
+inline OneofDef::const_iterator OneofDef::begin() const {
+  return const_iterator(this);
+}
+inline OneofDef::const_iterator OneofDef::end() const {
+  return const_iterator::end(this);
+}
+
+inline OneofDef::iterator::iterator(OneofDef* o) {
+  upb_oneof_begin(&iter_, o);
+}
+inline OneofDef::iterator OneofDef::iterator::end(OneofDef* o) {
+  OneofDef::iterator iter(o);
+  upb_oneof_iter_setdone(&iter.iter_);
+  return iter;
+}
+inline FieldDef* OneofDef::iterator::operator*() const {
+  return upb_oneof_iter_field(&iter_);
+}
+inline void OneofDef::iterator::operator++() { return upb_oneof_next(&iter_); }
+inline bool OneofDef::iterator::operator==(const iterator &other) const {
+  return upb_inttable_iter_isequal(&iter_, &other.iter_);
+}
+inline bool OneofDef::iterator::operator!=(const iterator &other) const {
+  return !(*this == other);
+}
+
+inline OneofDef::const_iterator::const_iterator(const OneofDef* md) {
+  upb_oneof_begin(&iter_, md);
+}
+inline OneofDef::const_iterator OneofDef::const_iterator::end(
+    const OneofDef *md) {
+  OneofDef::const_iterator iter(md);
+  upb_oneof_iter_setdone(&iter.iter_);
+  return iter;
+}
+inline const FieldDef* OneofDef::const_iterator::operator*() const {
+  return upb_msg_iter_field(&iter_);
+}
+inline void OneofDef::const_iterator::operator++() {
+  return upb_oneof_next(&iter_);
+}
+inline bool OneofDef::const_iterator::operator==(
+    const const_iterator &other) const {
+  return upb_inttable_iter_isequal(&iter_, &other.iter_);
+}
+inline bool OneofDef::const_iterator::operator!=(
+    const const_iterator &other) const {
+  return !(*this == other);
+}
+
 }  // namespace upb
 #endif
 
diff --git a/upb/handlers.c b/upb/handlers.c
index c0fd271a6f..fd19633c33 100644
--- a/upb/handlers.c
+++ b/upb/handlers.c
@@ -40,8 +40,10 @@ static void freehandlers(upb_refcounted *r) {
 static void visithandlers(const upb_refcounted *r, upb_refcounted_visit *visit,
                           void *closure) {
   const upb_handlers *h = (const upb_handlers*)r;
-  upb_msg_iter i;
-  for(upb_msg_begin(&i, h->msg); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, h->msg);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     if (!upb_fielddef_issubmsg(f)) continue;
     const upb_handlers *sub = upb_handlers_getsubhandlers(h, f);
@@ -70,8 +72,10 @@ static upb_handlers *newformsg(const upb_msgdef *m, const void *owner,
 
   // For each submessage field, get or create a handlers object and set it as
   // the subhandlers.
-  upb_msg_iter i;
-  for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, m);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     if (!upb_fielddef_issubmsg(f)) continue;
 
@@ -428,8 +432,10 @@ bool upb_handlers_freeze(upb_handlers *const*handlers, int n, upb_status *s) {
 
     // Check that there are no closure mismatches due to missing Start* handlers
     // or subhandlers with different type-level types.
-    upb_msg_iter j;
-    for(upb_msg_begin(&j, h->msg); !upb_msg_done(&j); upb_msg_next(&j)) {
+    upb_msg_field_iter j;
+    for(upb_msg_field_begin(&j, h->msg);
+        !upb_msg_field_done(&j);
+        upb_msg_field_next(&j)) {
 
       const upb_fielddef *f = upb_msg_iter_field(&j);
       if (upb_fielddef_isseq(f)) {
diff --git a/upb/json/printer.c b/upb/json/printer.c
index b996ccfae9..3dd3c20f0f 100644
--- a/upb/json/printer.c
+++ b/upb/json/printer.c
@@ -437,9 +437,9 @@ void printer_sethandlers(const void *closure, upb_handlers *h) {
     }                                                              \
     break;
 
-  upb_msg_iter i;
-  upb_msg_begin(&i, upb_handlers_msgdef(h));
-  for(; !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  upb_msg_field_begin(&i, upb_handlers_msgdef(h));
+  for(; !upb_msg_field_done(&i); upb_msg_field_next(&i)) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
 
     upb_handlerattr name_attr = UPB_HANDLERATTR_INITIALIZER;
diff --git a/upb/pb/compile_decoder.c b/upb/pb/compile_decoder.c
index 11aa4e9447..377cbb497a 100644
--- a/upb/pb/compile_decoder.c
+++ b/upb/pb/compile_decoder.c
@@ -762,8 +762,10 @@ static void compile_method(compiler *c, upb_pbdecodermethod *method) {
   putsel(c, OP_STARTMSG, UPB_STARTMSG_SELECTOR, h);
  label(c, LABEL_FIELD);
   uint32_t* start_pc = c->pc;
-  upb_msg_iter i;
-  for(upb_msg_begin(&i, md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, md);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
     upb_fieldtype_t type = upb_fielddef_type(f);
 
@@ -813,9 +815,11 @@ static void find_methods(compiler *c, const upb_handlers *h) {
   newmethod(h, c->group);
 
   // Find submethods.
-  upb_msg_iter i;
+  upb_msg_field_iter i;
   const upb_msgdef *md = upb_handlers_msgdef(h);
-  for(upb_msg_begin(&i, md); !upb_msg_done(&i); upb_msg_next(&i)) {
+  for(upb_msg_field_begin(&i, md);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
     const upb_handlers *sub_h;
     if (upb_fielddef_type(f) == UPB_TYPE_MESSAGE &&
@@ -857,7 +861,7 @@ static void set_bytecode_handlers(mgroup *g) {
 }
 
 
-/* JIT setup. ******************************************************************/
+/* JIT setup. *****************************************************************/
 
 #ifdef UPB_USE_JIT_X64
 
diff --git a/upb/pb/encoder.c b/upb/pb/encoder.c
index d2c22e99f0..d5685dc0ff 100644
--- a/upb/pb/encoder.c
+++ b/upb/pb/encoder.c
@@ -378,8 +378,10 @@ static void newhandlers_callback(const void *closure, upb_handlers *h) {
   upb_handlers_setendmsg(h, endmsg, NULL);
 
   const upb_msgdef *m = upb_handlers_msgdef(h);
-  upb_msg_iter i;
-  for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, m);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     const upb_fielddef *f = upb_msg_iter_field(&i);
     bool packed = upb_fielddef_isseq(f) && upb_fielddef_isprimitive(f) &&
                   upb_fielddef_packed(f);
diff --git a/upb/pb/textprinter.c b/upb/pb/textprinter.c
index 8ceed68628..45c5e434c3 100644
--- a/upb/pb/textprinter.c
+++ b/upb/pb/textprinter.c
@@ -261,8 +261,10 @@ static void onmreg(const void *c, upb_handlers *h) {
   upb_handlers_setstartmsg(h, textprinter_startmsg, NULL);
   upb_handlers_setendmsg(h, textprinter_endmsg, NULL);
 
-  upb_msg_iter i;
-  for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
+  upb_msg_field_iter i;
+  for(upb_msg_field_begin(&i, m);
+      !upb_msg_field_done(&i);
+      upb_msg_field_next(&i)) {
     upb_fielddef *f = upb_msg_iter_field(&i);
     upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
     upb_handlerattr_sethandlerdata(&attr, f);
diff --git a/upb/symtab.c b/upb/symtab.c
index f172aaa500..817a581bed 100644
--- a/upb/symtab.c
+++ b/upb/symtab.c
@@ -139,8 +139,10 @@ static bool upb_resolve_dfs(const upb_def *def, upb_strtable *addtab,
     // For messages, continue the recursion by visiting all subdefs.
     const upb_msgdef *m = upb_dyncast_msgdef(def);
     if (m) {
-      upb_msg_iter i;
-      for(upb_msg_begin(&i, m); !upb_msg_done(&i); upb_msg_next(&i)) {
+      upb_msg_field_iter i;
+      for(upb_msg_field_begin(&i, m);
+          !upb_msg_field_done(&i);
+          upb_msg_field_next(&i)) {
         upb_fielddef *f = upb_msg_iter_field(&i);
         if (!upb_fielddef_hassubdef(f)) continue;
         // |= to avoid short-circuit; we need its side-effects.
@@ -293,8 +295,10 @@ bool upb_symtab_add(upb_symtab *s, upb_def *const*defs, int n, void *ref_donor,
     // Type names are resolved relative to the message in which they appear.
     const char *base = upb_msgdef_fullname(m);
 
-    upb_msg_iter j;
-    for(upb_msg_begin(&j, m); !upb_msg_done(&j); upb_msg_next(&j)) {
+    upb_msg_field_iter j;
+    for(upb_msg_field_begin(&j, m);
+        !upb_msg_field_done(&j);
+        upb_msg_field_next(&j)) {
       upb_fielddef *f = upb_msg_iter_field(&j);
       const char *name = upb_fielddef_subdefname(f);
       if (name && !upb_fielddef_subdef(f)) {
diff --git a/upb/table.int.h b/upb/table.int.h
index e851e903c3..e27fb01a05 100644
--- a/upb/table.int.h
+++ b/upb/table.int.h
@@ -207,6 +207,9 @@ typedef struct {
 #define UPB_STRTABLE_INIT(count, mask, ctype, size_lg2, entries) \
   {{count, mask, ctype, size_lg2, entries}}
 
+#define UPB_EMPTY_STRTABLE_INIT(ctype)                           \
+  UPB_STRTABLE_INIT(0, 0, ctype, 0, NULL)
+
 typedef struct {
   upb_table t;              // For entries that don't fit in the array part.
   const _upb_value *array;  // Array part of the table.  See const note above.