It could be the cylinder head gasket. Check by taking the oil filler cap off and looking for white sludge. This means that water is getting into the oil. If it's just a small amount of oil in the radiator and the car isnt losing power, I'd change the radiator antifreeze and fill the oil to max on the dipstick.Then check the radiator and oil level regularly to see if the oil level drops or has water in it. I'd stick to short journeys for the time being.