I have oil in the radiator. What would cause this?

Auto, no oil cooler

Now,oil cooler,if equiped,is an integral part of the radiator. Are you sure it does not have one? Auot trans. has two lines going to the rad. located on the passenger side. Oil cooler has two larger lines going to the rad.on the drivers side. If NO oil cooler, most LIKELY suspect is trans.cooler which is also an integral part of the radiator. Either case susspect RAD.