Never- - - -ever- - - -use an O ring on a distributor, even on the point type if they have a groove on the housing. Yours is not the first "no oil pressure" experience I've heard of caused by an O ring. On the original equipment distributors, an O ring can create just enough drag that the vacuum advance can't move the housing quickly enough to respond to manifold vacuum changes. That can cause a detonation rattle while accelerating.
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!