The groove in the housing is there to keep liquid oil from getting splashed onto the housing and creeping up to overflow. No O ring is needed. If there's enough blowby to force oil past the groove, the engine has serious problems. At any speed above 30 MPH, there's enough airflow past the end of the road draft tube to create a slight negative pressure in the crankcase.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!