First, make sure your flame trap is clean, and that the three hoses leading to/from the flame trap housing are clear.

A blockage in this system will cause a buildup of crankcase oil pressure, with leaks resulting wherever the resistance is least. If there is a blockage, and you seal up your head as others have described, you will next get oil appearing somewhere else. Not to mention that the crankcase has become a home for foul vapors and their fallout.

Checking the large hose headed down to the breather box is a bit of a pain. Maybe pull the other two hoses at their far ends, seal off one, and blow into the other. If the oil filler cap is removed, you should be able to blow through. If you can't, there's a blockage below the flame trap housing and you'll need to clear it.