Why is there coolant in the oil? ?

Subaru's are very well known for issues with head gaskets. As such, the symptoms you are describing can only be from one of two things, or a combination of both. Either you have a head gasket failure (vehicles can still pass the standard hydrocarbon block test which is designed for determining if there are exhaust emissions in cooling system), or you have a crack in the engine block - more specifically, in the cylinder wall, which may have caused a crack to form in the steel head gasket material. Another way to inspect for a head gasket failure is to pressurize each cylinder with a leak down tester kit (with all valves closed) and check to see if coolant level rises in the radiator. If it does, tear down is required to completely determine level of failure.