Omega is an abandoned Element Zero mining station now inhabited by criminals, mercenaries and generally distasteful people. It is firmly under the control of the Asari Aria T'Loak.
Shepard travels to Omega to enlist Archangel (Garrus Vakarian) and Mordin Solus for his eventual attack on the Collector base. Zaeed is also recruited on Omega if the player owns his DLC. Samara's loyalty mission takes place on Omega. As it is a hub world, Omega also has a few side quests that can be completed.