I guess your dipstick is dual purpose, it's also a relief valve. When there's too much pressure for the crankcase vent system to recycle back into the intake, due to either blockage or blow-by being too great, something's gotta give.
From what I understand, the OAS acts as a PCV valve and is sealed by vacuum. So something screwy with the vacuum could make it stay closed. The crankcase is supposed to run under vacuum, not pressure. When you pull the dipstick or oil cap when running it should suck and rpm's change.
I guess you could take off the hose going into the OAS and see how much it's pumping, there's probably a way to measure the pressure, I just don't know what the normal measurement would be.