What Happens If Oil Gets Into Your Water In A Car?

If oil got into your water in your car I.E it is now in your radiator then more than likely you have a bad head gasket.  Most of the time this can be fixed by just replacing the head gasket.  If your engine overheated recently when the gasket first went bad then you may need to replace the entire head.  The reason the head may need replacing is that if the car overheated bad enough and the gasket went bad then the head may have cracked.  If that is the case then you can replace the head gasket but it will only be temporary and the gasket will eventually go bad again.  You can remove the head and take it to a machine shop for inspection.  I don't know if they can x-ray it or not.  The military has a section called "NDI", they can do a test to see if there are any cracks in any type of metal.  I don't know if a normal machine shop can do that or not.

Now if you or someone else actually put the oil in the radiator, then that isn't a big deal.  Just drain and flush the radiator and engine block until oil is gone and then refill radiator with water/antifreeze!
The only time that oil in the water is bad is when it happens on its own, this is a result of the head gasket going bad.  Oil is pumped throughout the engine block and head for lubrication and the water is pumped throughout the engine and head for coolant.  The head gasket separates the ports going from the engine block to the head so the water and oil stay separated.  

Hope this helps!!