Poids Mi-Moyens Hommes
 | Oleg Dadonov | 1-1-0 REC | #1813 |  |
 | Olendet Apouassa Romeo | 0-1-0 REC | #5725 |  |
 | Oliver Enkamp | 0-2-0 UFC | #3734 |  |
 | Olivier Fleurmon | 1-0-0 REC | #1951 |  |
 | Olli Santalahti | 2-3-0 REC | #3451 |  |
 | Omar El Dafrawy | 5-1-0 REC | #597 |  |
 | Omar Hussein | 0-2-0 REC | #3710 |  |
 | Omar Santana | 0-1-0 REC | #7280 |  |
 | Omari Akhmedov | 9-5-1 UFC | #592 | |
 | Omoyele Gonzalez | 0-1-0 REC | #5726 |  |
 | Omran Chaaban | 3-0-0 REC | #925 | |
 | Orion Cosce | 1-2-0 UFC | #3984 | |
 | Orlando Sanders | 0-1-0 REC | #7428 | |
 | Orlando Wiet | 1-1-0 UFC | #3078 | |
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #4624 | |
 | Oskar Staszczak | 1-0-0 REC | #2251 |  |
 | Oskar Szczepaniak | 4-2-0 REC | #2934 | |
 | Osvaldo Benedito | 0-1-0 REC | #5331 |  |
 | Oton Jasse | 0-3-0 REC | #4176 |  |
 | Oualy Tandia | 1-0-0 REC | #528 | |