| First | Last | Nickname | Ht. | Wt. | Reach | Stance | W | L | D | Belt |
|---|---|---|---|---|---|---|---|---|---|---|
| David | Onama | 5' 11" | 145 lbs. | 74.0" | Orthodox | 11 | 2 | 0 | ||
| Charlie | Ontiveros | American Bad Boy | 6' 2" | 155 lbs. | 74.0" | Switch | 11 | 9 | 0 | |
| Chibwikem | Onyenegecha | 6' 3" | 185 lbs. | -- | Orthodox | 5 | 0 | 0 | ||
| Myktybek | Orolbai | 5' 10" | 155 lbs. | 74.0" | Orthodox | 12 | 1 | 1 | ||
| Sam | Oropeza | 6' 2" | 170 lbs. | -- | 13 | 3 | 0 | |||
| Brian | Ortega | T-City | 5' 8" | 145 lbs. | 69.0" | Switch | 16 | 3 | 0 | |
| Tito | Ortiz | The People's Champ | 6' 3" | 205 lbs. | 74.0" | Orthodox | 19 | 12 | 1 | |
| Jorge | Ortiz | The Naked Man | 6' 0" | 185 lbs. | -- | Orthodox | 18 | 9 | 1 | |
| Dustin | Ortiz | 5' 5" | 125 lbs. | 65.0" | Orthodox | 19 | 8 | 0 | ||
| Kenji | Osawa | 5' 7" | 135 lbs. | 66.0" | Orthodox | 20 | 11 | 2 | ||
| Ode | Osbourne | The Jamaican Sensation | 5' 7" | 125 lbs. | 73.0" | Southpaw | 12 | 7 | 0 | |
| Kurt | Osiander | -- | 185 lbs. | -- | 4 | 1 | 0 | |||
| Nick | Osipczak | 6' 2" | 170 lbs. | 76.0" | Orthodox | 6 | 3 | 0 | ||
| Nissen | Osterneck | 6' 2" | 185 lbs. | -- | Orthodox | 12 | 4 | 0 | ||
| Rachael | Ostovich | 5' 3" | 125 lbs. | 62.0" | Orthodox | 4 | 6 | 0 | ||
| Pedro | Otavio | Te Pedro | 6' 3" | 245 lbs. | -- | Orthodox | 19 | 8 | 0 | |
| Alexander | Otsuka | The Diet Butcher | 6' 1" | 205 lbs. | -- | Orthodox | 5 | 13 | 0 | |
| Takafumi | Otsuka | 5' 6" | 139 lbs. | -- | Orthodox | 22 | 13 | 2 | ||
| Zak | Ottow | The Barbarian | 5' 11" | 170 lbs. | 72.0" | Orthodox | 17 | 7 | 0 | |
| Artur | Oumakhanov | 5' 7" | 154 lbs. | -- | Orthodox | 9 | 6 | 0 | ||
| Sidney | Outlaw | 5' 8" | 170 lbs. | 74.0" | Orthodox | 8 | 4 | 0 | ||
| Alistair | Overeem | The Demolition Man | 6' 4" | 265 lbs. | 80.0" | Orthodox | 47 | 19 | 0 | |
| Valentijn | Overeem | 6' 3" | 230 lbs. | -- | Orthodox | 30 | 32 | 0 | ||
| Craig | Oxley | 5' 9" | 155 lbs. | -- | 0 | 3 | 0 |