Sorted alphabetically by last name · 2711 fighters
| Name | Nickname | |
|---|---|---|
| OS | Ollie Schmid | "The Hungarian Stallion" |
| OB | Oluwale Bamgbose | "Holy War Angel" |
| OM | Omar Morales | "Venezuelan Fighter" |
| OA | Omari Akhmedov | "Wolverine" |
| OP | Onassis Parungao | |
| OC | Orion Cosce | "Galaxy" |
| OW | Orlando Wiet | "The Gladiator" |
| OP | Oskar Piechota | "Imadlo" |
| OT | Otari Tanzilovi | |
| OA | Ottman Azaitar | "Bulldozer" |