My vote is for the head gasket. This has happened to me twice although my engine is of slightly higher compression. Your local garage should have a combustion gas tester that they can replace the water cap with to check this. This has a chemical inside it which will change colour if exhaust gas is present in the coolant.

I never saw oil in the coolant, the fire rings failed allowing combustion gas into the waterways. Do your valve seals (and probably your guides to at that mileage with the head off) too and remember to check your head for flatness.