Tous les combattants
 | Omar Morales | 3-4-0 UFC | #2408 |  |
 | Omar Reguigui | 0-1-0, 1NC REC | #4545 |  |
 | Omar Santana | 0-1-0 REC | #7092 |  |
 | Omar Tugarev | 0-2-0 REC | #4060 |  |
 | Omari Akhmedov | 9-5-1 UFC | #563 |  |
 | Omiel Brown | 9-2-0 REC | #277 |  |
 | Omoyele Gonzalez | 0-1-0 REC | #5555 |  |
 | Omran Chaaban | 3-0-0 REC | #598 | |
 | Onassis Parungao | 1-0-0 UFC | #6098 |  |
 | Ondrej Petrasek | 1-1-0 REC | #6499 |  |
 | Ondrej Raska | 4-2-0 REC | #1135 | |
 | Orion Cosce | 1-2-0 UFC | #3866 | |
 | Orlando Sanders | 0-1-0 REC | #7192 | |
 | Orlando Wiet | 1-1-0 UFC | #2949 |  |
 | Orlando Wilson Prins | 2-4-0 REC | #2998 |  |
 | Ornella Vivaldi | 0-1-0 REC | #7645 | |
 | Oron Kahlon | 0-1-0 REC | #5238 |  |
 | Ortsa Gudaev | 1-1-0 REC | #4834 | |
 | Osama Elsaidy | 0-1-0 REC | #6844 |  |
 | Oscar Jurado Sanchez | 1-0-0 REC | #778 | |
 | Oscar Mendez | 0-1-0 REC | #5457 | |
 | Oscar Miguel | #7524 |  | |
 | Oscar Ownsworth | 1-3-0 REC | #3411 | |
| Oseas Cruz Furtado | 0-1-0 REC | #7620 |  |
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #4469 | |