diff --git a/gapic/schema/api.py b/gapic/schema/api.py
index df3e1daa8e..3c79d8f7cd 100644
--- a/gapic/schema/api.py
+++ b/gapic/schema/api.py
@@ -615,7 +615,7 @@ def _get_fields(self,
         # `_load_message` method.
         answer: Dict[str, wrappers.Field] = collections.OrderedDict()
         for i, field_pb in enumerate(field_pbs):
-            is_oneof = oneofs and field_pb.oneof_index > 0
+            is_oneof = oneofs and field_pb.HasField('oneof_index')
             oneof_name = nth(
                 (oneofs or {}).keys(),
                 field_pb.oneof_index
diff --git a/gapic/schema/wrappers.py b/gapic/schema/wrappers.py
index 1061620378..bbeeec679b 100644
--- a/gapic/schema/wrappers.py
+++ b/gapic/schema/wrappers.py
@@ -239,6 +239,15 @@ def __hash__(self):
         # Identity is sufficiently unambiguous.
         return hash(self.ident)
 
+    def oneof_fields(self, include_optional=False):
+        oneof_fields = collections.defaultdict(list)
+        for field in self.fields.values():
+            # Only include proto3 optional oneofs if explicitly looked for.
+            if field.oneof and not field.proto3_optional or include_optional:
+                oneof_fields[field.oneof].append(field)
+
+        return oneof_fields
+
     @utils.cached_property
     def field_types(self) -> Sequence[Union['MessageType', 'EnumType']]:
         answer = tuple(
@@ -583,6 +592,15 @@ def client_output(self):
     def client_output_async(self):
         return self._client_output(enable_asyncio=True)
 
+    def flattened_oneof_fields(self, include_optional=False):
+        oneof_fields = collections.defaultdict(list)
+        for field in self.flattened_fields.values():
+            # Only include proto3 optional oneofs if explicitly looked for.
+            if field.oneof and not field.proto3_optional or include_optional:
+                oneof_fields[field.oneof].append(field)
+
+        return oneof_fields
+
     def _client_output(self, enable_asyncio: bool):
         """Return the output from the client layer.
 
@@ -685,6 +703,10 @@ def filter_fields(sig: str) -> Iterable[Tuple[str, Field]]:
 
         return answer
 
+    @utils.cached_property
+    def flattened_field_to_key(self):
+        return {field.name: key for key, field in self.flattened_fields.items()}
+
     @utils.cached_property
     def legacy_flattened_fields(self) -> Mapping[str, Field]:
         """Return the legacy flattening interface: top level fields only,
diff --git a/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2 b/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2
index 4f30579121..322094d226 100644
--- a/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2
+++ b/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2
@@ -288,9 +288,15 @@ def test_{{ method.name|snake_case }}(transport: str = 'grpc'):
         call.return_value = iter([{{ method.output.ident }}()])
         {% else -%}
         call.return_value = {{ method.output.ident }}(
-            {%- for field in method.output.fields.values() | rejectattr('message')%}{% if not (field.oneof and not field.proto3_optional) %}
+          {%- for field in method.output.fields.values() | rejectattr('message')%}{% if not field.oneof or field.proto3_optional %}
             {{ field.name }}={{ field.mock_value }},
             {% endif %}{%- endfor %}
+            {#- This is a hack to only pick one field  #}
+            {%- for oneof_fields in method.output.oneof_fields().values() %}
+            {% with field = oneof_fields[0] %}
+            {{ field.name }}={{ field.mock_value }},
+            {%- endwith %}
+            {%- endfor %}
         )
         {% endif -%}
         {% if method.client_streaming %}
@@ -567,9 +573,15 @@ def test_{{ method.name|snake_case }}_flattened():
         # request object values.
         assert len(call.mock_calls) == 1
         _, args, _ = call.mock_calls[0]
-        {% for key, field in method.flattened_fields.items() -%}
+        {% for key, field in method.flattened_fields.items() -%}{%- if not field.oneof or field.proto3_optional %}
         assert args[0].{{ key }} == {{ field.mock_value }}
-        {% endfor %}
+        {% endif %}{% endfor %}
+        {%- for oneofs in method.flattened_oneof_fields().values() %}
+        {%- with field = oneofs[-1]  %}
+        assert args[0].{{ method.flattened_field_to_key[field.name] }} == {{ field.mock_value }}
+        {%- endwith %}
+        {%- endfor %}
+
 
 
 def test_{{ method.name|snake_case }}_flattened_error():
@@ -640,9 +652,14 @@ async def test_{{ method.name|snake_case }}_flattened_async():
         # request object values.
         assert len(call.mock_calls)
         _, args, _ = call.mock_calls[0]
-        {% for key, field in method.flattened_fields.items() -%}
+        {% for key, field in method.flattened_fields.items() -%}{%- if not field.oneof or field.proto3_optional %}
         assert args[0].{{ key }} == {{ field.mock_value }}
-        {% endfor %}
+        {% endif %}{% endfor %}
+        {%- for oneofs in method.flattened_oneof_fields().values() %}
+        {%- with field = oneofs[-1]  %}
+        assert args[0].{{ method.flattened_field_to_key[field.name] }} == {{ field.mock_value }}
+        {%- endwith %}
+        {%- endfor %}
 
 
 @pytest.mark.asyncio
diff --git a/tests/unit/schema/wrappers/test_message.py b/tests/unit/schema/wrappers/test_message.py
index 7ae95d0299..7d8cca169a 100644
--- a/tests/unit/schema/wrappers/test_message.py
+++ b/tests/unit/schema/wrappers/test_message.py
@@ -235,3 +235,26 @@ def test_field_map():
     entry_field = make_field('foos', message=entry_msg, repeated=True)
     assert entry_msg.map
     assert entry_field.map
+
+
+def test_oneof_fields():
+    mass_kg = make_field(name="mass_kg", oneof="mass", type=5)
+    mass_lbs = make_field(name="mass_lbs", oneof="mass", type=5)
+    length_m = make_field(name="length_m", oneof="length", type=5)
+    length_f = make_field(name="length_f", oneof="length", type=5)
+    color = make_field(name="color", type=5)
+    request = make_message(
+        name="CreateMolluscReuqest",
+        fields=(
+            mass_kg,
+            mass_lbs,
+            length_m,
+            length_f,
+            color,
+        ),
+    )
+    actual_oneofs = request.oneof_fields()
+    expected_oneofs = {
+        "mass": [mass_kg, mass_lbs],
+        "length": [length_m, length_f],
+    }
diff --git a/tests/unit/schema/wrappers/test_method.py b/tests/unit/schema/wrappers/test_method.py
index c0102402c2..f10bb078cd 100644
--- a/tests/unit/schema/wrappers/test_method.py
+++ b/tests/unit/schema/wrappers/test_method.py
@@ -364,3 +364,59 @@ def test_method_legacy_flattened_fields():
     ])
 
     assert method.legacy_flattened_fields == expected
+
+
+def test_flattened_oneof_fields():
+    mass_kg = make_field(name="mass_kg", oneof="mass", type=5)
+    mass_lbs = make_field(name="mass_lbs", oneof="mass", type=5)
+
+    length_m = make_field(name="length_m", oneof="length", type=5)
+    length_f = make_field(name="length_f", oneof="length", type=5)
+
+    color = make_field(name="color", type=5)
+    mantle = make_field(
+        name="mantle",
+        message=make_message(
+            name="Mantle",
+            fields=(
+                make_field(name="color", type=5),
+                mass_kg,
+                mass_lbs,
+            ),
+        ),
+    )
+    request = make_message(
+        name="CreateMolluscReuqest",
+        fields=(
+            length_m,
+            length_f,
+            color,
+            mantle,
+        ),
+    )
+    method = make_method(
+        name="CreateMollusc",
+        input_message=request,
+        signatures=[
+            "length_m,",
+            "length_f,",
+            "mantle.mass_kg,",
+            "mantle.mass_lbs,",
+            "color",
+        ]
+    )
+
+    expected = {"mass": [mass_kg, mass_lbs], "length": [length_m, length_f]}
+    actual = method.flattened_oneof_fields()
+    assert expected == actual
+
+    # Check this method too becasue the setup is a lot of work.
+    expected = {
+        "color": "color",
+        "length_m": "length_m",
+        "length_f": "length_f",
+        "mass_kg": "mantle.mass_kg",
+        "mass_lbs": "mantle.mass_lbs",
+    }
+    actual = method.flattened_field_to_key
+    assert expected == actual