The top 4 causes are probably what I listed. Unfortunately, outside of the oil cooler (if it has one) and maybe the head gasket, it may need to go to the shop.
If you just noticed it, how long could it have been there? How much oil is it? If it's something you don't check very often, I'd consider flushing the radiator/cooling system then make it a point to check regularly to see what comes back and how quickly.