diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index be99ab7683..c8830a6a9f 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -530,7 +530,7 @@ def _custom_auth(
 
     def _build_headers(self, options: FinalRequestOptions, *, retries_taken: int = 0) -> httpx2.Headers:
         custom_headers = options.headers or {}
-        headers_dict = _merge_mappings({**self._auth_headers(options.security), **self.default_headers}, custom_headers)
+        headers_dict = _merge_headers(self._auth_headers(options.security), self.default_headers, custom_headers)
         self._validate_headers(headers_dict, custom_headers)
 
         # headers are case-insensitive while dictionaries are not.
@@ -2342,3 +2342,17 @@ def _merge_mappings(
     """
     merged = {**obj1, **obj2}
     return {key: value for key, value in merged.items() if not isinstance(value, Omit)}
+
+
+def _merge_headers(*mappings: Headers) -> dict[str, str]:
+    """Merge headers case-insensitively, with later mappings taking precedence."""
+    merged: dict[str, tuple[str, str]] = {}
+    for mapping in mappings:
+        for name, value in mapping.items():
+            normalized_name = name.lower()
+            if isinstance(value, Omit):
+                merged.pop(normalized_name, None)
+            else:
+                merged[normalized_name] = (name, value)
+
+    return {name: value for name, value in merged.values()}
diff --git a/tests/test_client.py b/tests/test_client.py
index 01626384b4..6e8b4d2e72 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -156,6 +156,40 @@ def _get_open_connections(client: OpenAI | AsyncOpenAI) -> int:
     return len(cast(Any, transport)._pool._requests)
 
 
+@pytest.mark.parametrize("is_async", [False, True])
+@pytest.mark.parametrize(
+    "extra_headers,expected",
+    [
+        ({}, ["Bearer fake-default"]),
+        ({"AUTHORIZATION": "Bearer fake-request"}, ["Bearer fake-request"]),
+        ({"AUTHORIZATION": Omit()}, []),
+    ],
+    ids=["default", "override", "omit"],
+)
+async def test_case_insensitive_auth_headers(
+    is_async: bool, extra_headers: dict[str, str | Omit], expected: list[str]
+) -> None:
+    def handler(request: httpx2.Request) -> httpx2.Response:
+        assert request.headers.get_list("authorization") == expected
+        return httpx2.Response(200, json={"object": "list", "data": []})
+
+    transport = httpx2.MockTransport(handler)
+    if is_async:
+        async with AsyncOpenAI(
+            api_key="fake-original",
+            default_headers={"authorization": "Bearer fake-default"},
+            http_client=httpx2.AsyncClient(transport=transport),
+        ) as async_client:
+            await async_client.models.list(extra_headers=extra_headers)
+    else:
+        with OpenAI(
+            api_key="fake-original",
+            default_headers={"authorization": "Bearer fake-default"},
+            http_client=httpx2.Client(transport=transport),
+        ) as client:
+            client.models.list(extra_headers=extra_headers)
+
+
 class TestOpenAI:
     @pytest.mark.parametrize(
         "code_fields,expected_code",