First and foremost of the ancient Gallifreyans' Solar Engineers. Using his Stellar Manipulator (also known as the Hand of Omega), he detonated a star into a supernova, the tremendous energy of which was required for the Gallifreyans to achieve mastery of time travel and become Time Lords. However, when the supernova suddenly collapsed into a black hole, Omega was trapped and projected into an anti-matter universe. He blamed the Time Lords for his predicament, and swore revenge. When Omega tried to escape, the Time Lords summoned the Three Doctors to fight him. Omega then discovered that he could not leave the anti-matter universe, because anti-matter had totally destroyed his physical being, and only his mind was left. He was presumed dead in a matter/anti-matter explosion caused when his anti-matter being came into contact with the Second Doctor's positive matter recorder.
However, Omega survived and sought to return from the anti-matter universe. This time by using the Arc of Infinity, a collapsed Q-star to stabilise himself. He allied himself with a collaborator on Gallifrey, invaded the Matrix, and tried to bond physically with the Fifth Doctor, so that he could remain in our universe. But something went wrong and Omega's new body began to break down, reverting back to anti-matter. The Doctor was forced to use a matter converter to banish and, presumably, destroy Omega before a huge anti-matter explosion could annihilate Amsterdam.
If anything can be learnt from Omega's story it is that the Time Lords acted almost without compassion throughout. They abandoned one of their greatest scientific benefactors, risked the Doctor's lives to destroy him and were all too quick to execute the Doctor when they feared his body would form another bridge for Omega. While Omega was undeniably criminally unbalanced, his condition was perhaps unsurprising in light of his fellow Time Lords' behaviour.