If I'm counting right, there are actually only 12 Omnis that are also available in separate OHCs - Annihilation, AXM, Busiek's Avengers, Bendis' Daredevil, Hawkeye, New Avengers, New X-Men, Powers, Ultimates and Ultimate Spider-Man. Oh, and I guess the Ultimate sampler Omni, so 13 out of nearly 130 Omnibus volumes.