Tous les combattants
 | Omar Reguigui | 0-1-0, 1NC REC | #4626 |  |
 | Omar Santana | 0-1-0 REC | #7352 |  |
 | Omar Tugarev | 0-2-0 REC | #4428 |  |
 | Omari Akhmedov | 9-5-1 UFC | #590 |  |
 | Omiel Brown | 9-3-0 REC | #474 |  |
 | Omoyele Gonzalez | 0-1-0 REC | #5907 |  |
 | Omran Chaaban | 3-0-0 REC | #1170 | |
 | Onassis Parungao | 1-0-0 UFC | #6304 |  |
 | Ondrej Petrasek | 1-1-0 REC | #6871 |  |
 | Ondrej Raska | 4-3-0 REC | #1473 | |
 | Orion Cosce | 1-2-0 UFC | #3870 | |
 | Orlando Sanders | 0-1-0 REC | #7580 | |
 | Orlando Wiet | 1-1-0 UFC | #3143 |  |
 | Orlando Wilson Prins | 2-4-0 REC | #3282 |  |
 | Ornella Vivaldi | 0-1-0 REC | #7982 | |
 | Oron Kahlon | 0-1-0 REC | #5632 |  |
 | Ortsa Gudaev | 1-1-0 REC | #5262 | |
 | Osama Elsaidy | 0-1-0 REC | #7174 |  |
 | Oscar Jurado Sanchez | 2-0-0 REC | #296 | |
 | Oscar Mendez | 0-1-0 REC | #5179 | |
 | Oscar Miguel | #7882 |  | |
 | Oscar Ownsworth | 2-3-0 REC | #656 | |
 | Oscar Ravello | #8057 | | |
| Oseas Cruz Furtado | 0-1-0 REC | #7933 |  |
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #4783 | |