Poids Mi-Moyens Hommes
 | Oganes Voskanyan | 1-1-0 REC | #2308 |  |
 | Oleg Dadonov | 1-1-0 REC | #1417 | |
 | Olendet Apouassa Romeo | 0-1-0 REC | #5492 |  |
 | Oliver Enkamp | 0-2-0 UFC | #3568 |  |
 | Olivier Fleurmon | 1-0-0 REC | #1643 |  |
 | Olli Santalahti | 2-3-0 REC | #3267 |  |
 | Omar El Dafrawy | 5-1-0 REC | #355 |  |
 | Omar Hussein | 0-2-0 REC | #3648 |  |
 | Omar Santana | 0-1-0 REC | #7087 |  |
 | Omari Akhmedov | 9-5-1 UFC | #571 | |
 | Omoyele Gonzalez | 0-1-0 REC | #5609 |  |
 | Omran Chaaban | 3-0-0 REC | #559 | |
 | Orion Cosce | 1-2-0 UFC | #3854 | |
 | Orlando Sanders | 0-1-0 REC | #7182 | |
 | Orlando Wiet | 1-1-0 UFC | #2933 | |
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #4460 | |
 | Oskar Staszczak | 1-0-0 REC | #1929 |  |
 | Oskar Szczepaniak | 4-2-0 REC | #2770 | |
 | Osvaldo Benedito | 0-1-0 REC | #5374 |  |
 | Oton Jasse | 0-3-0 REC | #3963 |  |