Tous les combattants
 | Oluwale Bamgbose | 1-4-0 UFC | #3651 |  |
 | Olzhas Moldagaliev | #8075 |  | |
 | Omar Amasha | #7787 |  | |
 | Omar El Bily | 0-1-0 REC | #6106 |  |
 | Omar El Dafrawy | 6-1-0 REC | #393 | |
 | Omar Hussein | 1-2-0 REC | #728 |  |
 | Omar Morales | 3-4-0 UFC | #2550 |  |
 | Omar Reguigui | 0-1-0, 1NC REC | #4658 |  |
 | Omar Santana | 0-1-0 REC | #7353 |  |
 | Omar Tugarev | 0-2-0 REC | #4471 |  |
 | Omari Akhmedov | 9-5-1 UFC | #591 |  |
 | Omiel Brown | 9-3-0 REC | #519 |  |
 | Omoyele Gonzalez | 0-1-0 REC | #5994 | |
 | Omran Chaaban | 3-0-0 REC | #1230 | |
 | Onassis Parungao | 1-0-0 UFC | #6191 |  |
 | Ondrej Petrasek | 1-1-0 REC | #6918 |  |
 | Ondrej Raska | 4-3-0 REC | #1504 | |
 | Orion Cosce | 1-2-0 UFC | #3895 | |
 | Orlando Sanders | 1-1-0 REC | #7589 | |
 | Orlando Wiet | 1-1-0 UFC | #3121 |  |
 | Orlando Wilson Prins | 2-4-0 REC | #3302 |  |
 | Ornella Vivaldi | 0-1-0 REC | #8010 | |
 | Oron Kahlon | 0-1-0 REC | #5685 |  |
 | Ortsa Gudaev | 1-1-0 REC | #5333 | |
 | Osama Elsaidy | 0-1-0 REC | #7134 | |