Poids Mi-Moyens Hommes
 | Oganes Voskanyan | 1-2-0 REC | #2686 |  |
 | Oleg Dadonov | 1-1-0 REC | #2255 | |
 | Olendet Apouassa Romeo | 0-1-0 REC | #5762 |  |
 | Oliver Enkamp | 0-2-0 UFC | #3913 |  |
 | Olivier Fleurmon | 1-0-0 REC | #2344 |  |
 | Olli Santalahti | 2-3-0 REC | #3643 |  |
 | Omar El Dafrawy | 6-1-0 REC | #516 |  |
 | Omar Hussein | 1-2-0 REC | #923 |  |
 | Omar Santana | 0-1-0 REC | #7428 |  |
 | Omari Akhmedov | 9-5-1 UFC | #627 | |
 | Omoyele Gonzalez | 0-1-0 REC | #6144 |  |
 | Omran Chaaban | 3-0-0 REC | #1412 | |
 | Orion Cosce | 1-2-0 UFC | #3986 | |
 | Orlando Sanders | 1-1-0 REC | #767 | |
 | Orlando Wiet | 1-1-0 UFC | #3133 | |
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #4973 | |
 | Oskar Staszczak | 1-0-0 REC | #2706 |  |
 | Oskar Szczepaniak | 4-2-0 REC | #3050 | |
 | Osvaldo Benedito | 0-1-0 REC | #5282 |  |
 | Oton Jasse | 0-3-0 REC | #4514 |  |