It depends on how much oil you put in. If your gasoline tank is almost full and you put in just a little oil, your car will not notice much difference for a little while. The oil will mix with the gasoline and burn in the engine. Since the burning mixture will not be a clean burning, the spark plugs will begin to foul. The unburned oil (there will likely be some, will likely make some smoke exiting your tail pipe.
Let me shorten this answer and just say that it is not a good thing to do. It will harm your engine, leading to an expensive repair and it is not good for the environment either.
Let me shorten this answer and just say that it is not a good thing to do. It will harm your engine, leading to an expensive repair and it is not good for the environment either.