Omni-Man has been a hero for most of the Invincible series.
A lot of people consider Omni-Man to be the best Invincible villain, but surprisingly, he’s only been a villain for the first 12 issues.
After that he began his redemption arc which started when he went to a big planet where the aliens have a very short life span.
It was there that he learned that it’s not about how much time you have, but rather it’s about what you do with the time you DO have. He even had a child with one of those aliens.
After that he reconciles with Mark and does various things to make amends, one thing led to another, and through all that, Omni-Man has been doing his best to be a hero.
Omni man was hardly a villain,mark calls him out on that multiple times saying he doesn’t want to do this he feels like he doesn’t have a choice.
Some of the things in the fight between invincible and omni man doesn’t even happen like that train scene or city scene which nolan does to show mark that he’s right about humanity being fragile and weak.