Sorted alphabetically by last name · 2769 fighters
| Name | Nickname | |
|---|---|---|
| OT | Oleg Taktarov | "The Russian Bear" |
| OT | Oli Thompson | |
| OE | Oliver Enkamp | |
| OA | Olivier Aubin-Mercier | "The Canadian Gangster" |
| 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" |