Tous les combattants
 | Omar Hussein | 0-2-0 REC | #3496 |  |
 | Omar Morales | 3-4-0 UFC | #2339 |  |
 | Omar Reguigui | 0-1-0, 1NC REC | #4488 |  |
 | Omar Santana | 0-1-0 REC | #6975 |  |
 | Omar Tugarev | 0-2-0 REC | #3837 |  |
 | Omari Akhmedov | 9-5-1 UFC | #548 |  |
 | Omiel Brown | 9-2-0 REC | #168 |  |
 | Omoyele Gonzalez | 0-1-0 REC | #5350 |  |
 | Omran Chaaban | 2-0-0 REC | #4057 | |
 | Onassis Parungao | 1-0-0 UFC | #5982 |  |
 | Ondrej Petrasek | 1-1-0 REC | #6124 |  |
 | Ondrej Raska | 4-2-0 REC | #829 | |
 | Orion Cosce | 1-2-0 UFC | #3694 | |
 | Orlando Sanders | 0-1-0 REC | #6981 | |
 | Orlando Wiet | 1-1-0 UFC | #3073 |  |
 | Orlando Wilson Prins | 2-4-0 REC | #2839 |  |
 | Ornella Vivaldi | 0-1-0 REC | #7452 | |
 | Oron Kahlon | 0-1-0 REC | #5251 |  |
 | Ortsa Gudaev | 1-1-0 REC | #4573 | |
 | Osama Elsaidy | 0-1-0 REC | #6664 |  |
 | Oscar Jurado Sanchez | #7616 | | |
 | Oscar Mendez | 0-1-0 REC | #5203 | |
 | Oscar Miguel | #7297 |  | |
 | Oscar Ownsworth | 1-3-0 REC | #3216 | |
| Oseas Cruz Furtado | 0-1-0 REC | #7437 |  |