Wonder Woman's divine power, magical weapons, and combat skill give her a slight edge over Omni-Man's raw Viltrumite power.
Both are incredibly powerful warriors with centuries of combat experience. Omni-Man has Viltrumite physiology granting FTL speed and planet-cracking strength. Wonder Woman has divine Amazonian power, magical weapons, and has fought DC's most powerful beings.
Wonder Woman's Bracelets of Submission can block attacks from Superman-level threats, and her Lasso of Truth is unbreakable and could restrain Omni-Man. Her sword, forged by Hephaestus, can cut Superman and would certainly cut a Viltrumite. These magical weapons give her options beyond raw physical power.
Omni-Man's speed may match or slightly exceed Diana's, and his ruthlessness is an advantage against a warrior who fights with honor. But Wonder Woman has fought and defeated opponents as powerful as Omni-Man before, and her divine heritage grants resistance to physical damage that matches Viltrumite durability.
Omni-Man's FTL speed and planet-cracking strength may exceed Wonder Woman's physical stats. His ruthless willingness to kill immediately gives him an edge against an honorable warrior.
Wonder Woman's magical weapons bypass conventional durability, her divine heritage provides immense power, and her combat skill over thousands of years matches Omni-Man's. The Lasso of Truth is unbreakable.
Wonder Woman edges this out through magical weapons that bypass Viltrumite durability and divine power that matches Omni-Man's physical stats. It is a very close fight between two incredible warriors.
Omni-Man also fights
Wonder Woman also fights