Omni-ManvsSaitama

This is a clash between a realistically portrayed superhuman and a satirical character designed to win every fight. Omni-Man is genuinely one of the most powerful characters in his universe — FTL speed, planet-cracking strength, and millennia of experience. In a conventionally written fight, he would be a formidable challenge for almost anyone.

But Saitama exists as a narrative concept: he is the character who wins. His power is literally defined as being beyond whatever his opponent can do. In the manga, he has shown feats including punching through a planet-destroying attack, time travel through sheer punch power, and zero-diffing a cosmic-level threat. His serious punch generates more force than anything Omni-Man has tanked.

Omni-Man's experience and ruthlessness are real advantages, but Saitama's durability feats include tanking attacks that would atomize Omni-Man. The satirical nature of One Punch Man makes this difficult to judge by conventional power scaling, but taking both characters at face value of their feats, Saitama's ceiling is simply higher.