Tous les combattants
 | Oscar Ravello | #8082 |  | |
 | Oseas Cruz Furtado | 0-1-0 REC | #7818 |  |
 | Oseyiomon Oiyemhonlan | 1-2-0 REC | #4643 |  |
 | Osimkhon Rakhmonov | 2-1-0 REC | #2389 |  |
 | Oskar Piechota | 2-3-0, 1NC UFC | #3742 |  |
 | Oskar Stachura | 0-1-0 REC | #4993 | |
 | Oskar Staszczak | 1-0-0 REC | #2279 | |
 | Oskar Szczepaniak | 4-2-0 REC | #2946 | |
 | Osman Minbatirov | 1-1-0 REC | #5903 |  |
 | Osvaldo Benedito | 0-1-0 REC | #5336 |  |
 | Otabek Rajabov | 1-0-0 REC | #3383 | |
 | Otar Tanzilov | 0-1-0 REC | #4810 |  |
 | Otari Tanzilovi | 0-1-0 UFC | #4454 | |
 | Otgonbaatar Boldbaatar | 0-1-0 REC | #5681 |  |
 | Oton Jasse | 0-3-0 REC | #4206 | |
 | Ottman Azaitar | 2-3-0 UFC | #1880 |  |
 | Otto Rodrigues | 1-1-0 REC | #6474 | |
 | Oualy Tandia | 1-0-0 REC | #575 |  |
 | Oumar Kane | 1-0-0 REC | #2919 |  |
 | Oumar Sy | 3-2-0 UFC | #443 | |
 | Ourouzmag Magkeyev | 0-1-0 REC | #4362 | |
| Ousmane Camara | 1-0-0 REC | #1179 | |
 | Ovidio Bojorquez | 0-1-0 REC | #6899 |  |
 | Ovince St. Preux | 15-13-0 UFC | #279 | |
 | Owain Williams | 0-1-0 REC | #7017 |  |