diff --git a/src/agents/models/openai_chatcompletions.py b/src/agents/models/openai_chatcompletions.py
index 85adc81a1e..dcdfbfbd17 100644
--- a/src/agents/models/openai_chatcompletions.py
+++ b/src/agents/models/openai_chatcompletions.py
@@ -5,7 +5,7 @@
 from collections.abc import AsyncIterator
 from typing import TYPE_CHECKING, Any, Literal, cast, overload
 
-from openai import AsyncOpenAI, AsyncStream, Omit, omit
+from openai import AsyncOpenAI, AsyncStream, NotGiven, Omit, omit
 from openai.types import ChatModel
 from openai.types.chat import ChatCompletion, ChatCompletionChunk, ChatCompletionMessage
 from openai.types.chat.chat_completion import Choice
@@ -423,8 +423,11 @@ async def _fetch_response(
             "extra_body": model_settings.extra_body,
             "metadata": self._non_null_or_omit(model_settings.metadata),
         }
+        extra_args = model_settings.extra_args or {}
         duplicate_extra_arg_keys = sorted(
-            set(create_kwargs).intersection(model_settings.extra_args or {})
+            k
+            for k in extra_args
+            if k in create_kwargs and not isinstance(create_kwargs[k], Omit | NotGiven)
         )
         if duplicate_extra_arg_keys:
             if len(duplicate_extra_arg_keys) == 1:
@@ -436,7 +439,7 @@ async def _fetch_response(
             raise TypeError(
                 f"chat.completions.create() got multiple values for keyword arguments {keys}"
             )
-        create_kwargs.update(model_settings.extra_args or {})
+        create_kwargs.update(extra_args)
 
         ret = await self._get_client().chat.completions.create(**create_kwargs)
 
diff --git a/tests/models/test_openai_chatcompletions.py b/tests/models/test_openai_chatcompletions.py
index b2f8affd60..52e88db811 100644
--- a/tests/models/test_openai_chatcompletions.py
+++ b/tests/models/test_openai_chatcompletions.py
@@ -426,6 +426,28 @@ def __init__(self, completions: DummyCompletions) -> None:
     assert kwargs["stream_options"] is omit
 
 
+@pytest.mark.allow_call_model_methods
+@pytest.mark.asyncio
+async def test_fetch_response_allows_extra_arg_when_explicit_arg_is_omitted() -> None:
+    """A user-supplied extra_arg whose first-class field is only present as the
+    `omit` sentinel should pass through, not raise a duplicate-argument error."""
+    kwargs = await _run_chat_completions_model_with_custom_base_url(
+        model_settings=ModelSettings(extra_args={"temperature": 0.25})
+    )
+    assert kwargs["temperature"] == 0.25
+
+
+@pytest.mark.allow_call_model_methods
+@pytest.mark.asyncio
+async def test_fetch_response_rejects_duplicate_extra_args_with_explicit_value() -> None:
+    """When the first-class field has an explicit (non-omit) value, a colliding
+    extra_arg key should still raise a TypeError to surface the conflict."""
+    with pytest.raises(TypeError, match="multiple values.*temperature"):
+        await _run_chat_completions_model_with_custom_base_url(
+            model_settings=ModelSettings(temperature=0.5, extra_args={"temperature": 0.25})
+        )
+
+
 @pytest.mark.allow_call_model_methods
 @pytest.mark.asyncio
 async def test_custom_base_url_prompt_cache_key_uses_model_settings_only() -> None: