Tous les combattants
 | Olivier Fleurmon | 1-0-0 REC | #2027 |  |
 | Olivier Murad | 1-0-0 REC | #5458 |  |
 | Olli Santalahti | 2-3-0 REC | #3505 |  |
 | Ollie Sarwa | 5-1-0 REC | #323 |  |
 | Ollie Schmid | 0-1-0 UFC | #3161 |  |
 | Olutobi Kalejaiye | 1-1-0 REC | #1473 |  |
 | Oluwale Bamgbose | 1-4-0 UFC | #3603 |  |
 | Omar Amasha | #7724 |  | |
 | Omar El Bily | 0-1-0 REC | #6044 |  |
 | Omar El Dafrawy | 5-1-0 REC | #705 | |
 | Omar Hussein | 1-2-0 REC | #465 |  |
 | Omar Morales | 3-4-0 UFC | #2534 |  |
 | Omar Reguigui | 0-1-0, 1NC REC | #4594 |  |
 | Omar Santana | 0-1-0 REC | #7311 |  |
 | Omar Tugarev | 0-2-0 REC | #4332 | |
 | Omari Akhmedov | 9-5-1 UFC | #577 |  |
 | Omiel Brown | 9-3-0 REC | #407 | |
 | Omoyele Gonzalez | 0-1-0 REC | #5809 | |
 | Omran Chaaban | 3-0-0 REC | #1029 | |
 | Onassis Parungao | 1-0-0 UFC | #6166 |  |
 | Ondrej Petrasek | 1-1-0 REC | #6769 |  |
 | Ondrej Raska | 4-3-0 REC | #1385 | |
 | Orion Cosce | 1-2-0 UFC | #3992 | |
 | Orlando Sanders | 0-1-0 REC | #7499 | |
 | Orlando Wiet | 1-1-0 UFC | #3121 | |