I have a kawasaki KX250 motorcycle and there is oil in the radiator.

Check your engine and transmission oil for anti freeze. Also check to make sure your engine breather is not plugged. You probably have a leaking water pump seal , that is allowing oil to get into your anti freeze through the transmission. Normally when this happens the anti freeze goes into the transmission oil.
If there is no anti freeze in the lower engine or transmission , then the oil is leaking from a high pressure oil line going to the head to the cooling chambers filled with anti freeze. This means you probably need to replace your head gasket on the motor. While you have the head off check to make sure that the head has not warped. If it is warped it may have to be machined back to flat or replaced.