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!!