Warning: contains spoilers for the Invincible comic series!
Omni-Man, an immensely-powerful being based on Superman, has skyrocketed in popularity with the debut of Invincible, but as is the case for any sufficiently powerful character, fans have begun debating whether the Viltrumite powerhouse could take Dragon Ball's Saiyan warrior Goku. So, based on the comics and manga in which these characters originally appeared, which uber-powerful character would be able to defeat the other in a one-on-one battle?
However you slice it, Goku is a considerably older character than Omni-Man, first appearing in 1984 in a serialized story within Weekly Shonen Jump. In contrast, Omni-Man's debut was in Invincible #1, which arrived on shelves in January of 2003. Goku is very much a heroic character, and Omni-Man is a villainous mass-murderer who originally intends to conquer the planet for his alien race, so far from being a hypothetical clash, the two would likely come to blows if they found themselves in the same reality (Invincible actually includes its fair share of reality-hopping, with the titular hero even visiting the Marvel Universe at one point.)
Omni-Man is a Viltrumite from Viltrum, and as such possesses immense physical strength (enough to kill the Guardians of the Globe, the Invincible universe's Justice League, in literal seconds.) He can fly at supersonic speeds, punch holes through mountains, and easily take the full force of a train at high speed without so much as a scratch. Indeed, Omni-Man's durability is only rivaled by his longevity: Viltrumites can live for thousands of years, and Omni-Man has trained under the tutelage of the Viltrumite military to hone his fighting skills over time. The only enemy that ever defeated him was Thragg, another Viltrumite, and even then, only after a long, protracted battle. Omni-Man sees the people of Earth as insignificant insects who live and die in the blink of an eye. This worldview has, on occasion, colored his perception of those he judges inferior, and he has a tendency to underestimate unknown opponents.
Goku, like Omni-Man, is also an alien: he's a Saiyan from Vegeta, a planet with ten times the gravity of Earth. This grants Goku and all others of his race immense superhuman strength and durability. Goku has also trained from an early age to become a fierce fighter, with durability and strength equal to Omni-Man (if not outright surpassing it.) But Goku also possesses a unique power against which Omni-Man has no answer: Ki manipulation. By drawing Ki (the life force of every being) from inside himself, Goku can project energy, most famously as the Kamehameha. This wave of energy, if properly charged up, has enough power to destroy mountains, entire landmasses, and even the planet itself. The furthest extent of this power notwithstanding, Goku possesses a significant long-range attack, meaning Omni-Man must close the gap in order to take Goku down - but with the Kamehameha Wave and other Ki techniques, Omni-Man would be wise to keep his distance, not reduce it.
It's important to note that Goku has actually canonically died at least twice: once at the hands of Piccolo, and again while fighting Cell. He was resurrected each time, and the same cannot be said for Omni-Man: after his death towards the end of the Invincible comic run, he did not come back. Combine this with Goku's strength, durability, and speed, his powerful long-range attacks, and Omni-Man's penchant for underestimating his enemies, and Omni-Man fans are bound to be disappointed when Dragon Ball's Goku defeats him before the Invincible character can even land a punch.