Tous les combattants
 | Oluwale Bamgbose | 1-4-0 UFC | #3671 |  |
 | Olzhas Moldagaliev | #8091 |  | |
 | Omar Amasha | #7829 |  | |
 | Omar El Bily | 0-1-0 REC | #6152 |  |
 | Omar El Dafrawy | 6-1-0 REC | #429 | |
 | Omar Hussein | 1-2-0 REC | #779 |  |
 | Omar Morales | 3-4-0 UFC | #2562 |  |
 | Omar Reguigui | 0-1-0, 1NC REC | #4670 |  |
 | Omar Santana | 0-1-0 REC | #7360 |  |
 | Omar Tugarev | 0-2-0 REC | #4499 |  |
 | Omari Akhmedov | 9-5-1 UFC | #608 |  |
 | Omiel Brown | 9-3-0 REC | #540 |  |
 | Omoyele Gonzalez | 0-1-0 REC | #6042 | |
 | Omran Chaaban | 3-0-0 REC | #1271 | |
 | Onassis Parungao | 1-0-0 UFC | #6235 |  |
 | Ondrej Petrasek | 1-1-0 REC | #6970 |  |
 | Ondrej Raska | 4-3-0 REC | #1530 | |
 | Orion Cosce | 1-2-0 UFC | #3911 | |
 | Orlando Sanders | 1-1-0 REC | #587 | |
 | Orlando Wiet | 1-1-0 UFC | #3134 |  |
 | Orlando Wilson Prins | 2-4-0 REC | #3326 |  |
 | Ornella Vivaldi | 0-1-0 REC | #8044 | |
 | Oron Kahlon | 0-1-0 REC | #5603 |  |
 | Ortsa Gudaev | 1-1-0 REC | #5371 | |
 | Osama Elsaidy | 0-1-0 REC | #7152 | |