Which piston ring prevents combustion pressure from entering the crankcase?

The piston ring that prevents combustion pressure from entering the crankcase is the oil control ring. It helps maintain the proper amount of oil on the cylinder walls while also preventing excessive oil from entering the combustion chamber. The compression rings, located above the oil control ring, primarily seal the combustion chamber to contain combustion gases. However, it is the oil control ring that specifically addresses the prevention of combustion pressure leakage into the crankcase.