I have a 2000 VW Passat GLS 1.8l and it has oil in the radiator reservoir. and my Oil light says oil compression low. i flushed the radiator and found oil in it again, (the radiator overflow) it doesn't show the temp light anymore but now it shows the oil low compression light.
You probably have a blown head gasket. The only other explanation is the car has an oil to antifreeze cooler that is cracked or damaged. What is happening is that the oil pressure is greater than coolant pressure and is forcing it's way into the coolant.
The head gasket separates not only the combustion chamber but also coolant, pressurized oil and oil return. If the membrane between the coolant and pressurized oil is damaged these are the symptoms you would get. Because the oil pressure is escaping into the coolant that would cause low oil pressure.