So I take it this mower is new to you and the problem has existed since you've owned it?
O-rings are cheap. Go get the correct one and install it. Take a close look at the dipstick and the tube in which it resides. Figure out how it works, i.e., what is supposed to keep it locked in place. You may also need a new dipstick and/or tube.
Some engines (I can't say "all" because I haven't had them all apart) have a reed valve mechanism which acts as a check valve to relieve crankcase pressure and prevent the crankcase from refilling with air. This could also be a problem.