ProPokerTools Omaha Hi Simulation
600,000 trials (Randomized)
(350,917 wins, 2,319 ties)
(246,764 wins, 2,319 ties)
As you can see, hand values run much closer in Omaha. So in all-in pre-flop situations, yes, you will experience much more variance with that style of play.
May want to check this one out: http://www.cardschat.com/f57/plugging-leaks-omaha-171254/ it's not a super thread, but it'll get you started of you have more questions about Omaha.