Tous les combattants
 | Osimkhon Rakhmonov | 2-1-0 REC | #2098 |  |
 | Oskar Piechota | 2-3-0, 1NC UFC | #3581 |  |
 | Oskar Stachura | 0-1-0 REC | #4809 | |
 | Oskar Staszczak | 1-0-0 REC | #1957 | |
 | Oskar Szczepaniak | 4-2-0 REC | #2788 | |
 | Osman Minbatirov | 1-1-0 REC | #5724 |  |
 | Osvaldo Benedito | 0-1-0 REC | #5411 |  |
 | Otabek Rajabov | 1-0-0 REC | #3127 | |
 | Otar Tanzilov | 0-1-0 REC | #4773 |  |
 | Otgonbaatar Boldbaatar | #7705 |  | |
 | Oton Jasse | 0-3-0 REC | #3975 |  |
 | Ottman Azaitar | 2-3-0 UFC | #1794 |  |
 | Otto Rodrigues | 1-1-0 REC | #6165 | |
 | Oualy Tandia | #4410 |  | |
 | Oumar Kane | 1-0-0 REC | #2800 |  |
 | Oumar Sy | 3-2-0 UFC | #333 | |
 | Ourouzmag Magkeyev | 0-1-0 REC | #4202 | |
 | Ousmane Camara | 1-0-0 REC | #807 | |
 | Ovidio Bojorquez | 0-1-0 REC | #6705 |  |
 | Ovince St. Preux | 15-13-0 UFC | #270 |  |
 | Owain Williams | 0-1-0 REC | #6973 |  |
| Owen Jason | 1-0-0 REC | #7593 | |
 | Oyatullo Muminov | 2-1-0 REC | #629 | |
 | Ozan Aslaner | 3-2-0 REC | #713 |  |
 | Ozzy Diaz | 1-2-0 UFC | #1156 | |