Tous les combattants
 | Oskar Piechota | 2-3-0, 1NC UFC | #3945 |  |
 | Oskar Stachura | 1-1-0 REC | #1158 | |
 | Oskar Staszczak | 1-1-0 REC | #2437 | |
 | Oskar Szczepaniak | 4-2-0 REC | #3035 | |
 | Osman Minbatirov | 1-1-0 REC | #6100 |  |
 | Osvaldo Benedito | 0-1-0 REC | #5244 |  |
 | Otabek Rajabov | 1-0-0 REC | #3835 |  |
 | Otar Tanzilov | 0-1-0 REC | #5188 |  |
 | Otari Tanzilovi | 0-1-0 UFC | #3294 | |
 | Otgonbaatar Boldbaatar | 0-1-0 REC | #5807 |  |
 | Oton Jasse | 0-3-0 REC | #4639 |  |
 | Ottman Azaitar | 2-3-0 UFC | #1904 |  |
 | Otto Rodrigues | 1-1-0 REC | #6732 | |
 | Oualy Tandia | 1-0-0 REC | #1128 |  |
 | Oumar Kane | 1-0-0 REC | #3141 |  |
 | Oumar Sy | 3-3-0 UFC | #290 | |
 | Ourouzmag Magkeyev | 0-1-0 REC | #4399 | |
 | Ousmane Camara | 1-0-0 REC | #1856 | |
 | Oussama El Idrissi | #8116 | | |
 | Ovidio Bojorquez | 0-1-0 REC | #7419 |  |
 | Ovince St. Preux | 15-13-0 UFC | #307 |  |
 | Owain Williams | 0-1-0 REC | #7267 |  |
 | Owen Jason | 1-0-0 REC | #7563 | |
 | Oyatullo Muminov | 2-1-0 REC | #1718 | |
 | Ozan Aslaner | 3-2-0 REC | #1676 |  |