diff --git a/src/betterproto/__init__.py b/src/betterproto/__init__.py
index f7379098..fd9b32de 100644
--- a/src/betterproto/__init__.py
+++ b/src/betterproto/__init__.py
@@ -589,18 +589,20 @@ def __bytes__(self) -> bytes:
             # Being selected in a a group means this field is the one that is
             # currently set in a `oneof` group, so it must be serialized even
             # if the value is the default zero value.
-            selected_in_group = False
-            if meta.group and self._group_current[meta.group] == field_name:
-                selected_in_group = True
+            selected_in_group = (
+                meta.group and self._group_current[meta.group] == field_name
+            )
 
-            serialize_empty = False
-            if isinstance(value, Message) and value._serialized_on_wire:
-                # Empty messages can still be sent on the wire if they were
-                # set (or received empty).
-                serialize_empty = True
+            # Empty messages can still be sent on the wire if they were
+            # set (or received empty).
+            serialize_empty = isinstance(value, Message) and value._serialized_on_wire
+
+            include_default_value_for_oneof = self._include_default_value_for_oneof(
+                field_name=field_name, meta=meta
+            )
 
             if value == self._get_field_default(field_name) and not (
-                selected_in_group or serialize_empty
+                selected_in_group or serialize_empty or include_default_value_for_oneof
             ):
                 # Default (zero) values are not serialized. Two exceptions are
                 # if this is the selected oneof item or if we know we have to
@@ -629,6 +631,17 @@ def __bytes__(self) -> bytes:
                     sv = _serialize_single(2, meta.map_types[1], v)
                     output += _serialize_single(meta.number, meta.proto_type, sk + sv)
             else:
+                # If we have an empty string and we're including the default value for
+                # a oneof, make sure we serialize it. This ensures that the byte string
+                # output isn't simply an empty string. This also ensures that round trip
+                # serialization will keep `which_one_of` calls consistent.
+                if (
+                    isinstance(value, str)
+                    and value == ""
+                    and include_default_value_for_oneof
+                ):
+                    serialize_empty = True
+
                 output += _serialize_single(
                     meta.number,
                     meta.proto_type,
@@ -732,6 +745,13 @@ def _postprocess_single(
 
         return value
 
+    def _include_default_value_for_oneof(
+        self, field_name: str, meta: FieldMetadata
+    ) -> bool:
+        return (
+            meta.group is not None and self._group_current.get(meta.group) == field_name
+        )
+
     def parse(self: T, data: bytes) -> T:
         """
         Parse the binary encoded Protobuf into this message instance. This
@@ -810,10 +830,22 @@ def to_dict(
             cased_name = casing(field_name).rstrip("_")  # type: ignore
             if meta.proto_type == TYPE_MESSAGE:
                 if isinstance(value, datetime):
-                    if value != DATETIME_ZERO or include_default_values:
+                    if (
+                        value != DATETIME_ZERO
+                        or include_default_values
+                        or self._include_default_value_for_oneof(
+                            field_name=field_name, meta=meta
+                        )
+                    ):
                         output[cased_name] = _Timestamp.timestamp_to_json(value)
                 elif isinstance(value, timedelta):
-                    if value != timedelta(0) or include_default_values:
+                    if (
+                        value != timedelta(0)
+                        or include_default_values
+                        or self._include_default_value_for_oneof(
+                            field_name=field_name, meta=meta
+                        )
+                    ):
                         output[cased_name] = _Duration.delta_to_json(value)
                 elif meta.wraps:
                     if value is not None or include_default_values:
@@ -823,19 +855,28 @@ def to_dict(
                     value = [i.to_dict(casing, include_default_values) for i in value]
                     if value or include_default_values:
                         output[cased_name] = value
-                else:
-                    if value._serialized_on_wire or include_default_values:
-                        output[cased_name] = value.to_dict(
-                            casing, include_default_values
-                        )
-            elif meta.proto_type == TYPE_MAP:
+                elif (
+                    value._serialized_on_wire
+                    or include_default_values
+                    or self._include_default_value_for_oneof(
+                        field_name=field_name, meta=meta
+                    )
+                ):
+                    output[cased_name] = value.to_dict(casing, include_default_values,)
+            elif meta.proto_type == "map":
                 for k in value:
                     if hasattr(value[k], "to_dict"):
                         value[k] = value[k].to_dict(casing, include_default_values)
 
                 if value or include_default_values:
                     output[cased_name] = value
-            elif value != self._get_field_default(field_name) or include_default_values:
+            elif (
+                value != self._get_field_default(field_name)
+                or include_default_values
+                or self._include_default_value_for_oneof(
+                    field_name=field_name, meta=meta
+                )
+            ):
                 if meta.proto_type in INT_64_TYPES:
                     if field_is_repeated:
                         output[cased_name] = [str(n) for n in value]
@@ -894,6 +935,8 @@ def from_dict(self: T, value: dict) -> T:
                     elif meta.wraps:
                         setattr(self, field_name, value[key])
                     else:
+                        # NOTE: `from_dict` mutates the underlying message, so no
+                        # assignment here is necessary.
                         v.from_dict(value[key])
                 elif meta.map_types and meta.map_types[1] == TYPE_MESSAGE:
                     v = getattr(self, field_name)
@@ -919,8 +962,8 @@ def from_dict(self: T, value: dict) -> T:
                         elif isinstance(v, str):
                             v = enum_cls.from_string(v)
 
-                    if v is not None:
-                        setattr(self, field_name, v)
+                if v is not None:
+                    setattr(self, field_name, v)
         return self
 
     def to_json(self, indent: Union[None, int, str] = None) -> str:
diff --git a/tests/inputs/google_impl_behavior_equivalence/google_impl_behavior_equivalence.proto b/tests/inputs/google_impl_behavior_equivalence/google_impl_behavior_equivalence.proto
new file mode 100644
index 00000000..31b6bd3e
--- /dev/null
+++ b/tests/inputs/google_impl_behavior_equivalence/google_impl_behavior_equivalence.proto
@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+message Foo{
+  int64 bar = 1;
+}
+
+message Test{
+  oneof group{
+    string string = 1;
+    int64 integer = 2;
+    Foo foo = 3;
+  }
+}
diff --git a/tests/inputs/google_impl_behavior_equivalence/test_google_impl_behavior_equivalence.py b/tests/inputs/google_impl_behavior_equivalence/test_google_impl_behavior_equivalence.py
new file mode 100644
index 00000000..abe5d66d
--- /dev/null
+++ b/tests/inputs/google_impl_behavior_equivalence/test_google_impl_behavior_equivalence.py
@@ -0,0 +1,55 @@
+import pytest
+
+from google.protobuf import json_format
+import betterproto
+from tests.output_betterproto.google_impl_behavior_equivalence import (
+    Test,
+    Foo,
+)
+from tests.output_reference.google_impl_behavior_equivalence.google_impl_behavior_equivalence_pb2 import (
+    Test as ReferenceTest,
+    Foo as ReferenceFoo,
+)
+
+
+def test_oneof_serializes_similar_to_google_oneof():
+
+    tests = [
+        (Test(string="abc"), ReferenceTest(string="abc")),
+        (Test(integer=2), ReferenceTest(integer=2)),
+        (Test(foo=Foo(bar=1)), ReferenceTest(foo=ReferenceFoo(bar=1))),
+        # Default values should also behave the same within oneofs
+        (Test(string=""), ReferenceTest(string="")),
+        (Test(integer=0), ReferenceTest(integer=0)),
+        (Test(foo=Foo(bar=0)), ReferenceTest(foo=ReferenceFoo(bar=0))),
+    ]
+    for message, message_reference in tests:
+        # NOTE: As of July 2020, MessageToJson inserts newlines in the output string so,
+        # just compare dicts
+        assert message.to_dict() == json_format.MessageToDict(message_reference)
+
+
+def test_bytes_are_the_same_for_oneof():
+
+    message = Test(string="")
+    message_reference = ReferenceTest(string="")
+
+    message_bytes = bytes(message)
+    message_reference_bytes = message_reference.SerializeToString()
+
+    assert message_bytes == message_reference_bytes
+
+    message2 = Test().parse(message_reference_bytes)
+    message_reference2 = ReferenceTest()
+    message_reference2.ParseFromString(message_reference_bytes)
+
+    assert message == message2
+    assert message_reference == message_reference2
+
+    # None of these fields were explicitly set BUT they should not actually be null
+    # themselves
+    assert isinstance(message.foo, Foo)
+    assert isinstance(message2.foo, Foo)
+
+    assert isinstance(message_reference.foo, ReferenceFoo)
+    assert isinstance(message_reference2.foo, ReferenceFoo)
diff --git a/tests/inputs/oneof_default_value_serialization/oneof_default_value_serialization.proto b/tests/inputs/oneof_default_value_serialization/oneof_default_value_serialization.proto
new file mode 100644
index 00000000..44163c70
--- /dev/null
+++ b/tests/inputs/oneof_default_value_serialization/oneof_default_value_serialization.proto
@@ -0,0 +1,28 @@
+syntax = "proto3";
+
+import "google/protobuf/duration.proto";
+import "google/protobuf/timestamp.proto";
+import "google/protobuf/wrappers.proto";
+
+message Message{
+  int64 value = 1;
+}
+
+message NestedMessage{
+  int64 id = 1;
+  oneof value_type{
+    Message wrapped_message_value = 2;
+  }
+}
+
+message Test{
+  oneof value_type {
+      bool bool_value = 1;
+      int64 int64_value = 2;
+      google.protobuf.Timestamp timestamp_value = 3;
+      google.protobuf.Duration duration_value = 4;
+      Message wrapped_message_value = 5;
+      NestedMessage wrapped_nested_message_value = 6;
+      google.protobuf.BoolValue wrapped_bool_value = 7;
+  }
+}
\ No newline at end of file
diff --git a/tests/inputs/oneof_default_value_serialization/test_oneof_default_value_serialization.py b/tests/inputs/oneof_default_value_serialization/test_oneof_default_value_serialization.py
new file mode 100644
index 00000000..0c928cb8
--- /dev/null
+++ b/tests/inputs/oneof_default_value_serialization/test_oneof_default_value_serialization.py
@@ -0,0 +1,74 @@
+import pytest
+import datetime
+
+import betterproto
+from tests.output_betterproto.oneof_default_value_serialization import (
+    Test,
+    Message,
+    NestedMessage,
+)
+
+
+def assert_round_trip_serialization_works(message: Test) -> None:
+    assert betterproto.which_one_of(message, "value_type") == betterproto.which_one_of(
+        Test().from_json(message.to_json()), "value_type"
+    )
+
+
+def test_oneof_default_value_serialization_works_for_all_values():
+    """
+    Serialization from message with oneof set to default -> JSON -> message should keep
+    default value field intact.
+    """
+
+    test_cases = [
+        Test(bool_value=False),
+        Test(int64_value=0),
+        Test(
+            timestamp_value=datetime.datetime(
+                year=1970,
+                month=1,
+                day=1,
+                hour=0,
+                minute=0,
+                tzinfo=datetime.timezone.utc,
+            )
+        ),
+        Test(duration_value=datetime.timedelta(0)),
+        Test(wrapped_message_value=Message(value=0)),
+        # NOTE: Do NOT use betterproto.BoolValue here, it will cause JSON serialization
+        # errors.
+        # TODO: Do we want to allow use of BoolValue directly within a wrapped field or
+        # should we simply hard fail here?
+        Test(wrapped_bool_value=False),
+    ]
+    for message in test_cases:
+        assert_round_trip_serialization_works(message)
+
+
+def test_oneof_no_default_values_passed():
+    message = Test()
+    assert (
+        betterproto.which_one_of(message, "value_type")
+        == betterproto.which_one_of(Test().from_json(message.to_json()), "value_type")
+        == ("", None)
+    )
+
+
+def test_oneof_nested_oneof_messages_are_serialized_with_defaults():
+    """
+    Nested messages with oneofs should also be handled
+    """
+    message = Test(
+        wrapped_nested_message_value=NestedMessage(
+            id=0, wrapped_message_value=Message(value=0)
+        )
+    )
+    assert (
+        betterproto.which_one_of(message, "value_type")
+        == betterproto.which_one_of(Test().from_json(message.to_json()), "value_type")
+        == (
+            "wrapped_nested_message_value",
+            NestedMessage(id=0, wrapped_message_value=Message(value=0)),
+        )
+    )
diff --git a/tests/inputs/oneof_enum/test_oneof_enum.py b/tests/inputs/oneof_enum/test_oneof_enum.py
index fe21c435..e3eca13a 100644
--- a/tests/inputs/oneof_enum/test_oneof_enum.py
+++ b/tests/inputs/oneof_enum/test_oneof_enum.py
@@ -9,34 +9,36 @@
 from tests.util import get_test_case_json_data
 
 
-@pytest.mark.xfail
 def test_which_one_of_returns_enum_with_default_value():
     """
     returns first field when it is enum and set with default value
     """
     message = Test()
     message.from_json(get_test_case_json_data("oneof_enum", "oneof_enum-enum-0.json"))
-    assert message.move is None
+
+    assert message.move == Move(
+        x=0, y=0
+    )  # Proto3 will default this as there is no null
     assert message.signal == Signal.PASS
     assert betterproto.which_one_of(message, "action") == ("signal", Signal.PASS)
 
 
-@pytest.mark.xfail
 def test_which_one_of_returns_enum_with_non_default_value():
     """
     returns first field when it is enum and set with non default value
     """
     message = Test()
     message.from_json(get_test_case_json_data("oneof_enum", "oneof_enum-enum-1.json"))
-    assert message.move is None
-    assert message.signal == Signal.PASS
+    assert message.move == Move(
+        x=0, y=0
+    )  # Proto3 will default this as there is no null
+    assert message.signal == Signal.RESIGN
     assert betterproto.which_one_of(message, "action") == ("signal", Signal.RESIGN)
 
 
-@pytest.mark.xfail
 def test_which_one_of_returns_second_field_when_set():
     message = Test()
     message.from_json(get_test_case_json_data("oneof_enum"))
     assert message.move == Move(x=2, y=3)
-    assert message.signal == 0
+    assert message.signal == Signal.PASS
     assert betterproto.which_one_of(message, "action") == ("move", Move(x=2, y=3))
diff --git a/tests/test_features.py b/tests/test_features.py
index 0a8cb2d7..b5b38112 100644
--- a/tests/test_features.py
+++ b/tests/test_features.py
@@ -282,3 +282,38 @@ class TestParentMessage(betterproto.Message):
         "someDouble": 1.2,
         "someMessage": {"someOtherInt": 0},
     }
+
+
+def test_oneof_default_value_set_causes_writes_wire():
+    @dataclass
+    class Foo(betterproto.Message):
+        bar: int = betterproto.int32_field(1, group="group1")
+        baz: str = betterproto.string_field(2, group="group1")
+
+    def _round_trip_serialization(foo: Foo) -> Foo:
+        return Foo().parse(bytes(foo))
+
+    foo1 = Foo(bar=0)
+    foo2 = Foo(baz="")
+    foo3 = Foo()
+
+    assert bytes(foo1) == b"\x08\x00"
+    assert (
+        betterproto.which_one_of(foo1, "group1")
+        == betterproto.which_one_of(_round_trip_serialization(foo1), "group1")
+        == ("bar", 0)
+    )
+
+    assert bytes(foo2) == b"\x12\x00"  # Baz is just an empty string
+    assert (
+        betterproto.which_one_of(foo2, "group1")
+        == betterproto.which_one_of(_round_trip_serialization(foo2), "group1")
+        == ("baz", "")
+    )
+
+    assert bytes(foo3) == b""
+    assert (
+        betterproto.which_one_of(foo3, "group1")
+        == betterproto.which_one_of(_round_trip_serialization(foo3), "group1")
+        == ("", None)
+    )