Tous les combattants
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #4269 |  |
 | Osimkhon Rakhmonov | 2-0-0 REC | #2062 |  |
 | Oskar Piechota | 2-3-0, 1NC UFC | #3423 |  |
 | Oskar Stachura | 0-1-0 REC | #4614 | |
 | Oskar Staszczak | 1-0-0 REC | #1651 | |
 | Oskar Szczepaniak | 4-2-0 REC | #2608 | |
 | Osman Minbatirov | 1-1-0 REC | #5545 |  |
 | Osvaldo Benedito | 0-1-0 REC | #5148 |  |
 | Otabek Rajabov | 1-0-0 REC | #2898 | |
 | Otar Tanzilov | 0-1-0 REC | #4636 |  |
 | Oton Jasse | 0-3-0 REC | #3758 |  |
 | Ottman Azaitar | 2-3-0 UFC | #1788 |  |
 | Otto Rodrigues | 1-1-0 REC | #5921 | |
 | Oualy Tandia | #5115 |  | |
 | Oumar Kane | 1-0-0 REC | #2826 |  |
 | Oumar Sy | 3-2-0 UFC | #229 | |
 | Ourouzmag Magkeyev | 0-1-0 REC | #4105 | |
 | Ovidio Bojorquez | 0-1-0 REC | #6594 |  |
 | Ovince St. Preux | 15-13-0 UFC | #265 | |
 | Owain Williams | 0-1-0 REC | #6807 |  |
 | Owen Jason | 1-0-0 REC | #7394 | |
 | Oyatullo Muminov | 1-1-0 REC | #1341 | |
 | Ozan Aslaner | 2-2-0 REC | #1548 |  |
 | Ozzy Diaz | 1-1-0 UFC | #1824 | |
| Pablo David | 0-1-0 REC | #7424 | |