Tous les combattants
 | Omar Sargent | #8389 |  | |
 | Omar Tugarev | 0-2-0 REC | #4762 |  |
 | Omari Akhmedov | 9-5-1 UFC | #631 |  |
 | Omiel Brown | 9-3-0 REC | #657 | |
 | Omoyele Gonzalez | 0-1-0 REC | #6362 |  |
 | Omran Chaaban | 3-0-0 REC | #1564 | |
 | Onassis Parungao | 1-0-0 UFC | #6541 |  |
 | Ondrej Petrasek | 1-1-0 REC | #7262 |  |
 | Ondrej Raska | 4-3-0 REC | #1708 | |
 | Orion Cosce | 1-2-0 UFC | #4059 | |
 | Orlando Sanders | 1-1-0 REC | #930 | |
 | Orlando Wiet | 1-1-0 UFC | #2976 |  |
 | Orlando Wilson Prins | 2-4-0 REC | #3489 |  |
 | Ornella Vivaldi | 0-1-0 REC | #8209 | |
 | Oron Kahlon | 0-1-0 REC | #5538 |  |
 | Ortsa Gudaev | 1-1-0 REC | #5740 | |
 | Osama Elsaidy | 0-2-0 REC | #4475 |  |
 | Oscar Jurado Sanchez | 2-0-0 REC | #753 | |
 | Oscar Mendez | 0-1-0 REC | #5226 |  |
 | Oscar Miguel | #8125 |  | |
 | Oscar Ownsworth | 2-3-0 REC | #1112 | |
 | Oscar Ravello | 0-1-0 REC | #4668 | |
| Oseas Cruz Furtado | 0-1-0 REC | #8180 |  |
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #5139 | |
 | Osimkhon Rakhmonov | 3-1-0 REC | #715 |  |