diff --git a/litellm/llms/anthropic/common_utils.py b/litellm/llms/anthropic/common_utils.py
index 9461e40cf2e..1ed38946c59 100644
--- a/litellm/llms/anthropic/common_utils.py
+++ b/litellm/llms/anthropic/common_utils.py
@@ -78,8 +78,8 @@ def optionally_handle_anthropic_oauth(headers: dict, api_key: str | None) -> tup
     """
     Handle Anthropic OAuth token detection and header setup.
 
-    If an OAuth token is detected in the Authorization header, extracts it
-    and sets the required OAuth headers.
+    If an OAuth token is detected in the Authorization header (any casing),
+    extracts it and sets the required OAuth headers.
 
     Args:
         headers: Request headers dict
@@ -89,16 +89,18 @@ def optionally_handle_anthropic_oauth(headers: dict, api_key: str | None) -> tup
         Tuple of (updated headers, api_key)
     """
     # Check Authorization header (passthrough / forwarded requests)
-    auth_header: Final = headers.get("authorization", "")
-    if auth_header and auth_header.startswith(f"Bearer {ANTHROPIC_OAUTH_TOKEN_PREFIX}"):
-        api_key = auth_header.replace("Bearer ", "")
-        headers.pop("x-api-key", None)
+    auth_header: Final = next((value for name, value in headers.items() if name.lower() == "authorization"), "")
+    if auth_header.startswith(f"Bearer {ANTHROPIC_OAUTH_TOKEN_PREFIX}"):
+        api_key = auth_header.removeprefix("Bearer ")
+        for name in tuple(header_name for header_name in headers if header_name.lower() == "x-api-key"):
+            headers.pop(name)
         headers["anthropic-beta"] = _merge_beta_headers(headers.get("anthropic-beta"), ANTHROPIC_OAUTH_BETA_HEADER)
         headers["anthropic-dangerous-direct-browser-access"] = "true"
         return headers, api_key
     # Check api_key directly (standard chat/completion flow)
     if api_key and api_key.startswith(ANTHROPIC_OAUTH_TOKEN_PREFIX):
-        headers.pop("x-api-key", None)
+        for name in tuple(header_name for header_name in headers if header_name.lower() == "x-api-key"):
+            headers.pop(name)
         headers["authorization"] = f"Bearer {api_key}"
         headers["anthropic-beta"] = _merge_beta_headers(headers.get("anthropic-beta"), ANTHROPIC_OAUTH_BETA_HEADER)
         headers["anthropic-dangerous-direct-browser-access"] = "true"
diff --git a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
index 0a86bcfe85a..15780c9a160 100644
--- a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
+++ b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
@@ -31,13 +31,14 @@ FAKE_AUTH_TOKEN = "sk-ant-aut01-fake-auth-token-for-testing-123456789"
 class TestOptionallyHandleAnthropicOAuth:
     """Tests for optionally_handle_anthropic_oauth function."""
 
-    def test_oauth_token_in_authorization_header(self):
+    @pytest.mark.parametrize("header_name", ["authorization", "Authorization", "AUTHORIZATION"])
+    def test_oauth_token_in_authorization_header(self, header_name):
         """OAuth token in Authorization header should be detected and headers set correctly."""
         from litellm.llms.anthropic.common_utils import (
             optionally_handle_anthropic_oauth,
         )
 
-        headers = {"authorization": f"Bearer {FAKE_OAUTH_TOKEN}"}
+        headers = {header_name: f"Bearer {FAKE_OAUTH_TOKEN}"}
         updated_headers, extracted_api_key = optionally_handle_anthropic_oauth(
             headers, None
         )
@@ -47,6 +48,22 @@ class TestOptionallyHandleAnthropicOAuth:
         assert updated_headers["anthropic-dangerous-direct-browser-access"] == "true"
         assert "x-api-key" not in updated_headers
 
+    @pytest.mark.parametrize("api_key_header_name", ["x-api-key", "X-Api-Key"])
+    def test_oauth_removes_x_api_key_any_casing(self, api_key_header_name):
+        """When OAuth wins, a client x-api-key header is removed whatever its casing."""
+        from litellm.llms.anthropic.common_utils import (
+            optionally_handle_anthropic_oauth,
+        )
+
+        headers = {api_key_header_name: FAKE_REGULAR_KEY, "Authorization": f"Bearer {FAKE_OAUTH_TOKEN}"}
+        updated_headers, extracted_api_key = optionally_handle_anthropic_oauth(
+            headers, None
+        )
+
+        assert extracted_api_key == FAKE_OAUTH_TOKEN
+        assert [name for name in updated_headers if name.lower() == "x-api-key"] == []
+        assert updated_headers["Authorization"] == f"Bearer {FAKE_OAUTH_TOKEN}"
+
     def test_oauth_token_in_api_key_directly(self):
         """OAuth token passed as api_key should set Authorization: Bearer header."""
         from litellm.llms.anthropic.common_utils import (