1989 Festiva oil in radiator?

I have a 1989 Festiva, that has oil in the water in the radiator but no water in the oil in the oil pan. Does not smoke, runs fine, but a lot of oil in radiator and the over flow tank. What could cause this? I looked at the block and can't see any cracks visible, and looked at the head and don't see any. Reply as soon as possible please!