WARNING: The following article may contain major spoilers for the Invincible comic book series and potentially the upcoming animated series, debuting on Amazon Prime on March 26.
Over the course of Robert Kirkman, Cory Walker and Ryan Ottley's Invincible series, Omni-Man is undoubtedly one of the nastiest characters. Although he initially appeared to be a heroic, Superman-esque hero, Invincible's father was really hiding on Earth, supposedly pretending as if he really cared about Deborah and their son, Mark. In reality, he was prepping the planet for an invasion and takeover by the Viltrumites.
Despite all that, Nolan shockingly managed to redeem himself in Invincible #26, leaving Mark with the key insights he needed to end their destructive species.
Famously, Nolan pummelled Mark and was on the verge of killing him, but he was conflicted about his love for his son. He fled Earth and became a Viltrumite fugitive, hoping to find a place he could conquer and offer up as a substitute. Months later, Space Dog, an alien in disguise, would arrive and take Mark to Thraxa to save it from war, only for him to find his dad there ruling.
Instead of duking it out, the two supermen embraced, clearly missing each other. Nolan explained he found this planet a week after leaving Earth. After seeing how Mark loved Earth and wouldn't betray it, he too developed a sense of compassion and empathy. Nolan became monarch as these creatures only lived for nine months, so he turned into a pillar of stability for them, helping evolve the planet for the better.
To top it off, he also sired a kid, Oliver, with Andressa, and this is why he summoned Mark. He needed help since the Viltrumites were close. Nolan wanted to repent and save the world, and Mark was the perfect back-up. It was a shocking moment, leaving Invincible wondering if Omni-Man really made a face turn. He'd find out soon enough, though, as Lucan and two other soldiers came to kill Nolan for the empire.
It led to a brutal battle with Mark hiding Oliver and Andressa, all while trying to take out the soldiers. Lucan fell, but Mark was too weak to take on the other experienced ones. Still, he left in awe, seeing Nolan trying to save the aliens and also, watching his dad express sheer rage at Thraxa being dismantled. This was the humanity he always wanted from Nolan, but the price paid was steep as blood was shed everywhere.
This arc ended with the Viltrumites ripping Nolan's spine out and taking all their warriors to mend back home. Mark was offered leniency but with a condition: Nolan was going to be imprisoned and placed on trial for this betrayal, while Mark had to take his job on and infiltrate Earth, weakening it. Since Lucan was the only one who knew about Oliver, he had to protect his brother too. Most importantly, Nolan made up for the past by reminding Mark to read the books he wrote on his centuries of cosmic adventures, which Mark realized held the answers for him and other allies to destroy the war-hungry Viltrumites.