Actually there is a difference. OmniMax theatres are domed, 360 degree visually surrounded theatres, where IMAX theatres are simply really big flat screens. I've been to both, and while the IMAX is stunning, it doesn't compare to the realism of OMNIMAX. I remember actually getting woozy at times from vertigo and other sensations while watching the OMNIMAX movies. It's that realistic.
As far as I know, there's only a half a dozen or so Omnimax theatres in the world. (the one I saw was the science center in Michigan, USA. You'll know the difference, because the projector is actually about a dozen or so separate lenses that project a different part of the image on each area of the dome. And actually it's technically only about a 120 degree angle, though they don't bill it as such, however if you're sitting in your seat, it goes back as far as you can see. And these are specially made movies just for the omni max, because you have to film it with multiple cameras and convert it to the multi-camera projectors to view it on. That's why you'll never see mainstream movies there, just science and nature films... They're custom made.
EDIT:
Apparently IMAX and OMNIMAX films are produced by the same company. The difference still lies in the dome VS the flat screen.
http://www.msichicago.org/omax/aboutomni.html