fx140 oil in cylinders?

I put a FX 140 engine together and it ran good for a few minutes then died. I found motor oil in the breather
and in all four cylinders and lost quite a bit of oil from the tank. I have been trying to find the problem with not much luck. Looking in the repair manual I think I left out the dowel pin o-ring in the lower section of the crank case. Would that cause my problem?