Thor's divine power and Mjolnir give him a slight edge over Omni-Man's Viltrumite abilities.
Both are immensely powerful warriors with centuries of combat experience. Omni-Man is a Viltrumite who fights at FTL speeds and helped crack a planet. Thor is an Asgardian god who commands lightning, wields Mjolnir, and has strength approaching the Hulk's level.
The matchup is surprisingly close. Omni-Man may have a speed advantage โ Viltrumites routinely fight at FTL speeds, while Thor's combat speed, though impressive, is not consistently FTL. However, Thor's lightning attacks cover wide areas and travel at light speed, negating some of the speed difference.
Mjolnir is the deciding factor. The enchanted hammer hits with force that has staggered Galactus's heralds, and Thor's lightning can crack planets. Omni-Man's durability is enormous but he has been hurt by planet-level attacks. Thor at full power consistently operates at that level and beyond.
Omni-Man's FTL combat speed may exceed Thor's, and his Viltrumite durability has withstood planet-level impacts. His millennia of ruthless conquest experience give him a combat mentality advantage over the more honorable Thor.
Thor wields Mjolnir, commands divine lightning, and has strength rivaling the Hulk's. His godly nature grants resistance to physical damage, and his lightning attacks can blanket areas that negate speed advantages.
Thor edges this out through Mjolnir and divine lightning. Both are incredibly powerful warriors, but Thor's magical weapon and divine energy attacks give him slightly more firepower than Omni-Man's purely physical abilities.
Omni-Man also fights
Thor (Norse) also fights