The Omegas is/was a terrorist organization fighting for mutant supremacy over humans. They were founded and led by Callisto.
The Omegas were at a meeting discussing and arguing about Worthington Labs' mutant cure until Magneto steps in and recruits the Omegas into his Brotherhood.
While some of the Omegas died during the final battle on Alcatraz, many others were "depowered" by weaponized cure darts. Very many of them were lucky enough to escape the chaotic wrath of Phoenix.