Some possible causes of oil in the coolant system are a blown head gasket or a crack in a cylinder head. Another thing that can cause this problem is a cracked engine block.
A blown head gasket is the most common cause of oil getting into the coolant in an automobile. Head gaskets are often damaged due to the engine becoming overheated, though damage can also result from normal wear and tear. An internal crack in a cylinder head or in the engine block itself can also cause oil to contaminate the cooling system, although these are far less likely scenarios than a compromised head gasket. This problem should be addressed by a professional as soon as possible, since major engine damage can occur if left untreated.