If I got both {Helm of Obedience} and {Rest in Peace} in play, I can activate my helm unlimited times to mill my opponent's library as all cards going to his graveyard are removed from the game?
Because of the bonkers way that the Helm's effect is worded, yes. This does indeed work. Because none of the cards are ever reaching the graveyard, the Helm's conditions are never fully met, so it mills more in order to reach them. And more. And more.
Noteworthy: It doesn't work with any other mill effects, just that nutty helm.
Well that helm was really one of a kind. And costed quite a bit of its time.