diff --git a/php/ext/google/protobuf/message.c b/php/ext/google/protobuf/message.c
index 58511c2637..f38b326c0d 100644
--- a/php/ext/google/protobuf/message.c
+++ b/php/ext/google/protobuf/message.c
@@ -913,7 +913,7 @@ PHP_METHOD(Message, whichOneof) {
     return;
   }
 
-  field = upb_Message_WhichOneof(intern->msg, oneof);
+  field = upb_Message_WhichOneofByDef(intern->msg, oneof);
   RETURN_STRING(field ? upb_FieldDef_Name(field) : "");
 }
 
diff --git a/php/ext/google/protobuf/wkt.inc b/php/ext/google/protobuf/wkt.inc
index 4579c7e30d..799a48fd46 100644
--- a/php/ext/google/protobuf/wkt.inc
+++ b/php/ext/google/protobuf/wkt.inc
@@ -1350,7 +1350,7 @@ static PHP_METHOD(google_protobuf_Value, getKind) {
   const upb_OneofDef *oneof = upb_MessageDef_FindOneofByName(
       intern->desc->msgdef, "kind");
   const upb_FieldDef *field = 
-      upb_Message_WhichOneof(intern->msg, oneof);
+      upb_Message_WhichOneofByDef(intern->msg, oneof);
   RETURN_STRING(field ? upb_FieldDef_Name(field) : "");
 }
 static zend_function_entry google_protobuf_Value_phpmethods[] = {
diff --git a/python/message.c b/python/message.c
index d5213ab1af..42bf90236d 100644
--- a/python/message.c
+++ b/python/message.c
@@ -1039,9 +1039,9 @@ static PyObject* PyUpb_Message_HasField(PyObject* _self, PyObject* arg) {
 
   if (PyUpb_Message_IsStub(self)) Py_RETURN_FALSE;
 
-  return PyBool_FromLong(field ? upb_Message_HasFieldByDef(self->ptr.msg, field)
-                               : upb_Message_WhichOneof(self->ptr.msg, oneof) !=
-                                     NULL);
+  return PyBool_FromLong(
+      field ? upb_Message_HasFieldByDef(self->ptr.msg, field)
+            : upb_Message_WhichOneofByDef(self->ptr.msg, oneof) != NULL);
 }
 
 static PyObject* PyUpb_Message_Contains(PyObject* _self, PyObject* arg) {
@@ -1421,7 +1421,7 @@ static PyObject* PyUpb_Message_ClearField(PyObject* _self, PyObject* arg) {
     return NULL;
   }
 
-  if (o) f = upb_Message_WhichOneof(self->ptr.msg, o);
+  if (o) f = upb_Message_WhichOneofByDef(self->ptr.msg, o);
   if (f) PyUpb_Message_DoClearField(_self, f);
   Py_RETURN_NONE;
 }
@@ -1616,7 +1616,7 @@ static PyObject* PyUpb_Message_WhichOneof(PyObject* _self, PyObject* name) {
   }
   upb_Message* msg = PyUpb_Message_GetIfReified(_self);
   if (!msg) Py_RETURN_NONE;
-  const upb_FieldDef* f = upb_Message_WhichOneof(msg, o);
+  const upb_FieldDef* f = upb_Message_WhichOneofByDef(msg, o);
   if (!f) Py_RETURN_NONE;
   return PyUnicode_FromString(upb_FieldDef_Name(f));
 }
diff --git a/ruby/ext/google/protobuf_c/message.c b/ruby/ext/google/protobuf_c/message.c
index 706e3df5fe..acd900ffb7 100644
--- a/ruby/ext/google/protobuf_c/message.c
+++ b/ruby/ext/google/protobuf_c/message.c
@@ -248,7 +248,7 @@ static int extract_method_call(VALUE method_name, Message* self,
 static VALUE Message_oneof_accessor(VALUE _self, const upb_OneofDef* o,
                                     int accessor_type) {
   Message* self = ruby_to_Message(_self);
-  const upb_FieldDef* oneof_field = upb_Message_WhichOneof(self->msg, o);
+  const upb_FieldDef* oneof_field = upb_Message_WhichOneofByDef(self->msg, o);
 
   switch (accessor_type) {
     case METHOD_PRESENCE:
diff --git a/ruby/lib/google/protobuf/ffi/message.rb b/ruby/lib/google/protobuf/ffi/message.rb
index a3dc2d189b..40e33a313a 100644
--- a/ruby/lib/google/protobuf/ffi/message.rb
+++ b/ruby/lib/google/protobuf/ffi/message.rb
@@ -21,7 +21,7 @@ module Google
       attach_function :json_encode_message,     :upb_JsonEncode,              [:Message, Descriptor, :DefPool, :int, :binary_string, :size_t, Status.by_ref], :size_t
       attach_function :decode_message,          :upb_Decode,                  [:binary_string, :size_t, :Message, MiniTable.by_ref, :ExtensionRegistry, :int, Internal::Arena], DecodeStatus
       attach_function :get_mutable_message,     :upb_Message_Mutable,         [:Message, FieldDescriptor, Internal::Arena], MutableMessageValue.by_value
-      attach_function :get_message_which_oneof, :upb_Message_WhichOneof,      [:Message, OneofDescriptor], FieldDescriptor
+      attach_function :get_message_which_oneof, :upb_Message_WhichOneofByDef, [:Message, OneofDescriptor], FieldDescriptor
       attach_function :message_discard_unknown, :upb_Message_DiscardUnknown,  [:Message, Descriptor, :int], :bool
       attach_function :message_next,            :upb_Message_Next,            [:Message, Descriptor, :DefPool, :FieldDefPointer, MessageValue.by_ref, :pointer], :bool
       # MessageValue
diff --git a/src/google/protobuf/compiler/php/php_generator.cc b/src/google/protobuf/compiler/php/php_generator.cc
index 39944b59ed..006d6debd5 100644
--- a/src/google/protobuf/compiler/php/php_generator.cc
+++ b/src/google/protobuf/compiler/php/php_generator.cc
@@ -1758,7 +1758,7 @@ void GenerateCMessage(const Descriptor* message, io::Printer* printer) {
         "  const upb_OneofDef *oneof = upb_MessageDef_FindOneofByName(\n"
         "      intern->desc->msgdef, \"$name$\");\n"
         "  const upb_FieldDef *field = \n"
-        "      upb_Message_WhichOneof(intern->msg, oneof);\n"
+        "      upb_Message_WhichOneofByDef(intern->msg, oneof);\n"
         "  RETURN_STRING(field ? upb_FieldDef_Name(field) : \"\");\n"
         "}\n",
         "c_name", c_name, "name", oneof->name(), "camel_name",
diff --git a/upb/json/decode.c b/upb/json/decode.c
index c578451836..8fe655ccfd 100644
--- a/upb/json/decode.c
+++ b/upb/json/decode.c
@@ -957,7 +957,7 @@ static void jsondec_field(jsondec* d, upb_Message* msg,
   }
 
   if (upb_FieldDef_RealContainingOneof(f) &&
-      upb_Message_WhichOneof(msg, upb_FieldDef_ContainingOneof(f))) {
+      upb_Message_WhichOneofByDef(msg, upb_FieldDef_ContainingOneof(f))) {
     jsondec_err(d, "More than one field for this oneof.");
   }
 
diff --git a/upb/message/accessors.h b/upb/message/accessors.h
index 9e9b54b127..922026ee48 100644
--- a/upb/message/accessors.h
+++ b/upb/message/accessors.h
@@ -247,6 +247,12 @@ UPB_API_INLINE void* upb_Message_ResizeArrayUninitialized(
 UPB_API_INLINE uint32_t upb_Message_WhichOneofFieldNumber(
     const upb_Message* message, const upb_MiniTableField* oneof_field);
 
+// For a field `f` which is in a oneof, return the field of that
+// oneof that is actually set (or NULL if none).
+UPB_API_INLINE const upb_MiniTableField* upb_Message_WhichOneof(
+    const upb_Message* msg, const upb_MiniTable* m,
+    const upb_MiniTableField* f);
+
 // Updates a map entry given an entry message.
 bool upb_Message_SetMapEntry(upb_Map* map, const upb_MiniTable* mini_table,
                              const upb_MiniTableField* field,
diff --git a/upb/message/accessors_test.cc b/upb/message/accessors_test.cc
index 76248578b3..014056e9fd 100644
--- a/upb/message/accessors_test.cc
+++ b/upb/message/accessors_test.cc
@@ -500,4 +500,32 @@ TEST(GeneratedCode, OneofClear) {
   upb_Arena_Free(arena);
 }
 
+TEST(GeneratedCode, OneofAccess) {
+  upb_Arena* arena = upb_Arena_New();
+
+  protobuf_test_messages_proto2_TestAllTypesProto2* msg =
+      protobuf_test_messages_proto2_TestAllTypesProto2_new(arena);
+
+  const upb_MiniTable* table =
+      &protobuf_0test_0messages__proto2__TestAllTypesProto2_msg_init;
+
+  // oneof_uint32
+  const upb_MiniTableField* oneofField =
+      upb_MiniTable_FindFieldByNumber(table, 111);
+  EXPECT_TRUE(upb_MiniTableField_IsInOneof(oneofField));
+
+  const upb_MiniTableField* oneOfFirstFetch =
+      upb_Message_WhichOneof((upb_Message*)msg, table, oneofField);
+  // one of not set, so should initially yield nullptr
+  EXPECT_EQ(oneOfFirstFetch, nullptr);
+
+  protobuf_test_messages_proto2_TestAllTypesProto2_set_oneof_uint32(msg, 522);
+  const upb_MiniTableField* oneOfSecondFetch =
+      upb_Message_WhichOneof((upb_Message*)msg, table, oneofField);
+  // this oneof has now been set, so should yield the MiniTableField
+  EXPECT_EQ(oneOfSecondFetch, oneofField);
+
+  upb_Arena_Free(arena);
+}
+
 }  // namespace
diff --git a/upb/message/internal/accessors.h b/upb/message/internal/accessors.h
index d28e2ebfaf..5bbfb1e0bc 100644
--- a/upb/message/internal/accessors.h
+++ b/upb/message/internal/accessors.h
@@ -122,6 +122,17 @@ UPB_API_INLINE uint32_t upb_Message_WhichOneofFieldNumber(
   return UPB_PRIVATE(_upb_Message_GetOneofCase)(message, oneof_field);
 }
 
+UPB_API_INLINE const upb_MiniTableField* upb_Message_WhichOneof(
+    const struct upb_Message* msg, const upb_MiniTable* m,
+    const upb_MiniTableField* f) {
+  uint32_t field_number = upb_Message_WhichOneofFieldNumber(msg, f);
+  if (field_number == 0) {
+    // No field in the oneof is set.
+    return NULL;
+  }
+  return upb_MiniTable_FindFieldByNumber(m, field_number);
+}
+
 // LINT.ThenChange(GoogleInternalName2)
 
 // Returns false if the message is missing any of its required fields.
diff --git a/upb/reflection/message.c b/upb/reflection/message.c
index 9211738c33..285418295a 100644
--- a/upb/reflection/message.c
+++ b/upb/reflection/message.c
@@ -41,8 +41,8 @@ bool upb_Message_HasFieldByDef(const upb_Message* msg, const upb_FieldDef* f) {
   }
 }
 
-const upb_FieldDef* upb_Message_WhichOneof(const upb_Message* msg,
-                                           const upb_OneofDef* o) {
+const upb_FieldDef* upb_Message_WhichOneofByDef(const upb_Message* msg,
+                                                const upb_OneofDef* o) {
   const upb_FieldDef* f = upb_OneofDef_Field(o, 0);
   if (upb_OneofDef_IsSynthetic(o)) {
     UPB_ASSERT(upb_OneofDef_FieldCount(o) == 1);
diff --git a/upb/reflection/message.h b/upb/reflection/message.h
index b3b52d7ace..fbe34f4d04 100644
--- a/upb/reflection/message.h
+++ b/upb/reflection/message.h
@@ -30,8 +30,8 @@ UPB_API upb_MutableMessageValue upb_Message_Mutable(upb_Message* msg,
                                                     upb_Arena* a);
 
 // Returns the field that is set in the oneof, or NULL if none are set.
-UPB_API const upb_FieldDef* upb_Message_WhichOneof(const upb_Message* msg,
-                                                   const upb_OneofDef* o);
+UPB_API const upb_FieldDef* upb_Message_WhichOneofByDef(const upb_Message* msg,
+                                                        const upb_OneofDef* o);
 
 // Clear all data and unknown fields.
 void upb_Message_ClearByDef(upb_Message* msg, const upb_MessageDef* m);