i have oil in the header tank (water exspansion tank) i do not have over heating or running problems,what is wrong?

Good points by Tom, In addition to narrow it down even further there are a few things you can do. Check your oil for a white creamy-ness. Drain some oil into a pan and if there is water in the oil it will look brown and milky. With engine cool remove radiator cap and start car watch closely for bubbles coming up into filler neck. If they seem rythmatic and steady look for head/head gasket. You can do a pressure test on the radiator system.If there is a leakdown look for head/head gasket or bad radiator.

Am I gonna have to buy a new raditor