diff --git a/autogen/oai/client.py b/autogen/oai/client.py
index 5970c87126d4..19b937c0e2b4 100644
--- a/autogen/oai/client.py
+++ b/autogen/oai/client.py
@@ -10,7 +10,7 @@
 from typing import Protocol
 
 from autogen.cache.cache import Cache
-from autogen.oai.openai_utils import get_key, OAI_PRICE1K
+from autogen.oai.openai_utils import get_key, is_valid_api_key, OAI_PRICE1K
 from autogen.token_count_utils import count_token
 
 TOOL_ENABLED = False
@@ -47,6 +47,7 @@
 
 LEGACY_DEFAULT_CACHE_SEED = 41
 LEGACY_CACHE_DIR = ".cache"
+OPEN_API_BASE_URL_PREFIX = "https://api.openai.com"
 
 
 class ModelClient(Protocol):
@@ -111,6 +112,14 @@ class OpenAIClient:
 
     def __init__(self, client: Union[OpenAI, AzureOpenAI]):
         self._oai_client = client
+        if (
+            not isinstance(client, openai.AzureOpenAI)
+            and str(client.base_url).startswith(OPEN_API_BASE_URL_PREFIX)
+            and not is_valid_api_key(self._oai_client.api_key)
+        ):
+            logger.warning(
+                "The API key specified is not a valid OpenAI format; it won't work with the OpenAI-hosted model."
+            )
 
     def message_retrieval(
         self, response: Union[ChatCompletion, Completion]
diff --git a/autogen/oai/openai_utils.py b/autogen/oai/openai_utils.py
index 0407aa1cadf5..74bf81a401cf 100644
--- a/autogen/oai/openai_utils.py
+++ b/autogen/oai/openai_utils.py
@@ -1,6 +1,7 @@
 import json
 import logging
 import os
+import re
 import tempfile
 from pathlib import Path
 from typing import Any, Dict, List, Optional, Set, Union
@@ -74,6 +75,19 @@ def get_key(config: Dict[str, Any]) -> str:
     return json.dumps(config, sort_keys=True)
 
 
+def is_valid_api_key(api_key: str):
+    """Determine if input is valid OpenAI API key.
+
+    Args:
+        api_key (str): An input string to be validated.
+
+    Returns:
+        bool: A boolean that indicates if input is valid OpenAI API key.
+    """
+    api_key_re = re.compile(r"^sk-[A-Za-z0-9]{32,}$")
+    return bool(re.fullmatch(api_key_re, api_key))
+
+
 def get_config_list(
     api_keys: List, base_urls: Optional[List] = None, api_type: Optional[str] = None, api_version: Optional[str] = None
 ) -> List[Dict]:
diff --git a/test/agentchat/contrib/test_llava.py b/test/agentchat/contrib/test_llava.py
index 5ae5e4abb889..385077ffa4e4 100644
--- a/test/agentchat/contrib/test_llava.py
+++ b/test/agentchat/contrib/test_llava.py
@@ -5,6 +5,8 @@
 
 import autogen
 
+from conftest import MOCK_OPEN_AI_API_KEY
+
 try:
     from autogen.agentchat.contrib.llava_agent import (
         LLaVAAgent,
@@ -26,7 +28,7 @@ def setUp(self):
             llm_config={
                 "timeout": 600,
                 "seed": 42,
-                "config_list": [{"model": "llava-fake", "base_url": "localhost:8000", "api_key": "Fake"}],
+                "config_list": [{"model": "llava-fake", "base_url": "localhost:8000", "api_key": MOCK_OPEN_AI_API_KEY}],
             },
         )
 
@@ -103,7 +105,7 @@ def test_llava_call(self, mock_llava_call_binary, mock_llava_formatter):
 
         # Set up the llm_config dictionary
         llm_config = {
-            "config_list": [{"api_key": "value", "base_url": "localhost:8000"}],
+            "config_list": [{"api_key": MOCK_OPEN_AI_API_KEY, "base_url": "localhost:8000"}],
             "max_new_tokens": 2000,
             "temperature": 0.5,
             "seed": 1,
diff --git a/test/agentchat/contrib/test_lmm.py b/test/agentchat/contrib/test_lmm.py
index 866f8a7448ea..125b51464546 100644
--- a/test/agentchat/contrib/test_lmm.py
+++ b/test/agentchat/contrib/test_lmm.py
@@ -6,6 +6,8 @@
 import autogen
 from autogen.agentchat.conversable_agent import ConversableAgent
 
+from conftest import MOCK_OPEN_AI_API_KEY
+
 try:
     from autogen.agentchat.contrib.multimodal_conversable_agent import MultimodalConversableAgent
 except ImportError:
@@ -28,7 +30,7 @@ def setUp(self):
             llm_config={
                 "timeout": 600,
                 "seed": 42,
-                "config_list": [{"model": "gpt-4-vision-preview", "api_key": "sk-fake"}],
+                "config_list": [{"model": "gpt-4-vision-preview", "api_key": MOCK_OPEN_AI_API_KEY}],
             },
         )
 
diff --git a/test/agentchat/contrib/test_web_surfer.py b/test/agentchat/contrib/test_web_surfer.py
index abf7903dee7a..cfe6417ef329 100644
--- a/test/agentchat/contrib/test_web_surfer.py
+++ b/test/agentchat/contrib/test_web_surfer.py
@@ -7,7 +7,7 @@
 from autogen.cache import Cache
 
 sys.path.append(os.path.join(os.path.dirname(__file__), "../.."))
-from conftest import skip_openai  # noqa: E402
+from conftest import MOCK_OPEN_AI_API_KEY, skip_openai  # noqa: E402
 
 sys.path.append(os.path.join(os.path.dirname(__file__), ".."))
 from test_assistant_agent import KEY_LOC, OAI_CONFIG_LIST  # noqa: E402
@@ -48,7 +48,7 @@
 def test_web_surfer() -> None:
     with pytest.MonkeyPatch.context() as mp:
         # we mock the API key so we can register functions (llm_config must be present for this to work)
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         page_size = 4096
         web_surfer = WebSurferAgent(
             "web_surfer", llm_config={"config_list": []}, browser_config={"viewport_size": page_size}
diff --git a/test/agentchat/test_conversable_agent.py b/test/agentchat/test_conversable_agent.py
index 8ff8038da7e9..a10b1a0e3a07 100644
--- a/test/agentchat/test_conversable_agent.py
+++ b/test/agentchat/test_conversable_agent.py
@@ -15,7 +15,7 @@
 from autogen.agentchat import ConversableAgent, UserProxyAgent
 from autogen.agentchat.conversable_agent import register_function
 from test_assistant_agent import KEY_LOC, OAI_CONFIG_LIST
-from conftest import skip_openai
+from conftest import MOCK_OPEN_AI_API_KEY, skip_openai
 
 try:
     import openai
@@ -473,7 +473,7 @@ async def test_a_generate_reply_raises_on_messages_and_sender_none(conversable_a
 
 def test_update_function_signature_and_register_functions() -> None:
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         agent = ConversableAgent(name="agent", llm_config={})
 
         def exec_python(cell: str) -> None:
@@ -617,7 +617,7 @@ def get_origin(d: Dict[str, Callable[..., Any]]) -> Dict[str, Callable[..., Any]
 
 def test_register_for_llm():
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         agent3 = ConversableAgent(name="agent3", llm_config={"config_list": []})
         agent2 = ConversableAgent(name="agent2", llm_config={"config_list": []})
         agent1 = ConversableAgent(name="agent1", llm_config={"config_list": []})
@@ -690,7 +690,7 @@ async def exec_sh(script: Annotated[str, "Valid shell script to execute."]) -> s
 
 def test_register_for_llm_api_style_function():
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         agent3 = ConversableAgent(name="agent3", llm_config={"config_list": []})
         agent2 = ConversableAgent(name="agent2", llm_config={"config_list": []})
         agent1 = ConversableAgent(name="agent1", llm_config={"config_list": []})
@@ -761,7 +761,7 @@ async def exec_sh(script: Annotated[str, "Valid shell script to execute."]) -> s
 
 def test_register_for_llm_without_description():
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         agent = ConversableAgent(name="agent", llm_config={})
 
         with pytest.raises(ValueError) as e:
@@ -775,7 +775,7 @@ def exec_python(cell: Annotated[str, "Valid Python cell to execute."]) -> str:
 
 def test_register_for_llm_without_LLM():
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         agent = ConversableAgent(name="agent", llm_config=None)
         agent.llm_config = None
         assert agent.llm_config is None
@@ -791,7 +791,7 @@ def exec_python(cell: Annotated[str, "Valid Python cell to execute."]) -> str:
 
 def test_register_for_execution():
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         agent = ConversableAgent(name="agent", llm_config={"config_list": []})
         user_proxy_1 = UserProxyAgent(name="user_proxy_1")
         user_proxy_2 = UserProxyAgent(name="user_proxy_2")
@@ -826,7 +826,7 @@ async def exec_sh(script: Annotated[str, "Valid shell script to execute."]):
 
 def test_register_functions():
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         agent = ConversableAgent(name="agent", llm_config={"config_list": []})
         user_proxy = UserProxyAgent(name="user_proxy")
 
diff --git a/test/coding/test_commandline_code_executor.py b/test/coding/test_commandline_code_executor.py
index 1ee5e6be337e..ba0853d731b9 100644
--- a/test/coding/test_commandline_code_executor.py
+++ b/test/coding/test_commandline_code_executor.py
@@ -7,7 +7,7 @@
 from autogen.coding.local_commandline_code_executor import LocalCommandlineCodeExecutor
 from autogen.oai.openai_utils import config_list_from_json
 
-from conftest import skip_openai
+from conftest import MOCK_OPEN_AI_API_KEY, skip_openai
 
 
 def test_create() -> None:
@@ -152,7 +152,7 @@ def test_local_commandline_executor_conversable_agent_code_execution() -> None:
     with tempfile.TemporaryDirectory() as temp_dir:
         executor = LocalCommandlineCodeExecutor(work_dir=temp_dir)
         with pytest.MonkeyPatch.context() as mp:
-            mp.setenv("OPENAI_API_KEY", "mock")
+            mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
             _test_conversable_agent_code_execution(executor)
 
 
diff --git a/test/coding/test_embedded_ipython_code_executor.py b/test/coding/test_embedded_ipython_code_executor.py
index 3ecb318f959c..a43f103d4e24 100644
--- a/test/coding/test_embedded_ipython_code_executor.py
+++ b/test/coding/test_embedded_ipython_code_executor.py
@@ -7,7 +7,7 @@
 from autogen.coding.base import CodeBlock, CodeExecutor
 from autogen.coding.factory import CodeExecutorFactory
 from autogen.oai.openai_utils import config_list_from_json
-from conftest import skip_openai  # noqa: E402
+from conftest import MOCK_OPEN_AI_API_KEY, skip_openai  # noqa: E402
 
 try:
     from autogen.coding.embedded_ipython_code_executor import EmbeddedIPythonCodeExecutor
@@ -211,7 +211,7 @@ def test_function(a, b):
 ```
 """
     with pytest.MonkeyPatch.context() as mp:
-        mp.setenv("OPENAI_API_KEY", "mock")
+        mp.setenv("OPENAI_API_KEY", MOCK_OPEN_AI_API_KEY)
         reply = agent.generate_reply(
             [{"role": "user", "content": msg}],
             sender=ConversableAgent("user", llm_config=False, code_execution_config=False),
diff --git a/test/conftest.py b/test/conftest.py
index 266e18058e4d..e0469ea3ae07 100644
--- a/test/conftest.py
+++ b/test/conftest.py
@@ -4,6 +4,8 @@
 skip_redis = False
 skip_docker = False
 
+MOCK_OPEN_AI_API_KEY = "sk-mockopenaiAPIkeyinexpectedformatfortestingonly"
+
 
 # Registers command-line options like '--skip-openai' and '--skip-redis' via pytest hook.
 # When these flags are set, it indicates that tests requiring OpenAI or Redis (respectively) should be skipped.
diff --git a/test/oai/test_utils.py b/test/oai/test_utils.py
index 57a70c0ffee3..e10781a9cb78 100644
--- a/test/oai/test_utils.py
+++ b/test/oai/test_utils.py
@@ -8,7 +8,9 @@
 import pytest
 
 import autogen  # noqa: E402
-from autogen.oai.openai_utils import DEFAULT_AZURE_API_VERSION, filter_config
+from autogen.oai.openai_utils import DEFAULT_AZURE_API_VERSION, filter_config, is_valid_api_key
+
+from conftest import MOCK_OPEN_AI_API_KEY
 
 # Example environment variables
 ENV_VARS = {
@@ -370,5 +372,17 @@ def test_tags():
     assert len(list_5) == 0
 
 
+def test_is_valid_api_key():
+    assert not is_valid_api_key("")
+    assert not is_valid_api_key("sk-")
+    assert not is_valid_api_key("SK-")
+    assert not is_valid_api_key("sk-asajsdjsd2")
+    assert not is_valid_api_key("FooBar")
+    assert not is_valid_api_key("sk-asajsdjsd22372%23kjdfdfdf2329ffUUDSDS")
+    assert is_valid_api_key("sk-asajsdjsd22372X23kjdfdfdf2329ffUUDSDS")
+    assert is_valid_api_key("sk-asajsdjsd22372X23kjdfdfdf2329ffUUDSDS1212121221212sssXX")
+    assert is_valid_api_key(MOCK_OPEN_AI_API_KEY)
+
+
 if __name__ == "__main__":
     pytest.main()