Stars is going to be the only other site with lots and lots of omaha games.
However, if you are playing lower limits, then any of the sites with 10k-20k players during prime time should have games going.
I'd suggest checking out Absolute or Ultimate Bet after they merge (supposed to happen in days). That event will mean AP/UB will be pretty huge, and the Omaha tables should be active. Another site to check is Cake Poker. They've grown a LOT over the past two years, and supposedly it's one of the fishiest sites for holdem for USA players. Not sure on Omaha action, but the site is large enough in terms of population to support good low limit Omaha games now.
Again, if it's high roller action, then Stars and Full Tilt are really the only two with consistent high dollar Omaha or Stud games.
Oh, FYI - moved this over to the general chat area to get more views. Some of our resident Omaha players might be able to steer you better than I can.