Omni-Man is a planet-level threat whose power dwarfs Spider-Man's street-level abilities.
Spider-Man is one of Marvel's most beloved heroes, but Omni-Man operates at a completely different power level. Peter can lift about 10 tons; Omni-Man helped crack a planet. Spider-Man moves at supersonic speeds; Omni-Man fights at FTL. Every physical metric is a massive mismatch.
Spider-Man's spider-sense would alert him to Omni-Man's attacks, but sensing a planet-level punch does not help when you cannot survive it. Peter's agility could let him dodge for a short time, but Omni-Man's speed advantage means that window would close quickly.
Peter's intelligence and webbing are irrelevant against someone who can fly through mountains without slowing down. This is a matchup where Spider-Man's only option is to run, and Omni-Man is significantly faster.
Omni-Man's FTL speed and planet-cracking strength make Spider-Man's advantages in agility and spider-sense irrelevant. A single punch would be instantly fatal, and Peter cannot escape someone faster than light.
Spider-Man's spider-sense and agility let him dodge attacks from faster opponents, and his genius intellect has found solutions to seemingly impossible matchups before. His courage means he would never stop fighting.
Omni-Man wins instantly and decisively. The power gap between a street-level hero and a planet-cracking Viltrumite is so vast that no amount of spider-sense or cleverness can bridge it.
Omni-Man also fights
Spider-Man also fights