Sorted alphabetically by last name · 2769 fighters
| Name | Nickname | |
|---|---|---|
| OW | Orlando Wiet | "The Gladiator" |
| OP | Oskar Piechota | "Imadlo" |
| OT | Otari Tanzilovi | |
| OA | Ottman Azaitar | "Bulldozer" |
| OS | Oumar Sy | |
| OS | Ovince Saint Preux | |
| OD | Ozzy Diaz | |
| PG | Pablo Garza |
| "The Scarecrow" |
| PH | Paddy Holohan | "The Hooligan" |
| PP | Paddy Pimblett | "The Baddy" |