| First | Last | Nickname | Ht. | Wt. | Reach | Stance | W | L | D | Belt |
|---|---|---|---|---|---|---|---|---|---|---|
| Jake | O'Brien | Irish | 6' 3" | 205 lbs. | 76.0" | Orthodox | 15 | 4 | 0 | |
| TJ | O'Brien | The Spider | 6' 2" | 155 lbs. | -- | Orthodox | 20 | 8 | 0 | |
| Sean | O'Connell | The Real OC | 6' 1" | 205 lbs. | 74.0" | Orthodox | 17 | 9 | 0 | |
| Dan | O'Connor | Johnny Irish | 5' 4" | 115 lbs. | -- | 6 | 6 | 0 | ||
| Sean | O'Haire | 6' 5" | 270 lbs. | -- | Orthodox | 4 | 2 | 0 | ||
| Sean | O'Malley | Suga | 5' 11" | 135 lbs. | 72.0" | Switch | 18 | 3 | 0 | |
| Jeremiah | O'Neal | 5' 8" | 260 lbs. | -- | 13 | 25 | 0 | |||
| Chuck | O'Neil | Cold Steel | 6' 2" | 170 lbs. | -- | 17 | 9 | 0 | ||
| Casey | O'Neill | King | 5' 6" | 125 lbs. | 69.0" | Orthodox | 10 | 2 | 0 | |
| Brendan | O'Reilly | Badger | 5' 7" | 170 lbs. | 69.0" | Orthodox | 6 | 3 | 0 | |
| Takahiro | Oba | 5' 8" | 200 lbs. | -- | Southpaw | 5 | 7 | 1 | ||
| Nobuhiro | Obiya | 5' 8" | 145 lbs. | -- | Orthodox | 12 | 5 | 1 | ||
| Jose | Ochoa | Kalzifer | 5' 7" | 125 lbs. | 67.0" | Southpaw | 8 | 2 | 0 | |
| Christian | Ocon | 5' 8" | 135 lbs. | 69.0" | Orthodox | 2 | 1 | 0 | ||
| Richard | Odoms | The Black Eagle | 6' 5" | 265 lbs. | -- | Orthodox | 13 | 3 | 0 | |
| Volkan | Oezdemir | No Time | 6' 2" | 205 lbs. | 75.0" | Orthodox | 20 | 8 | 0 | |
| Kaan | Ofli | Genghis | 5' 7" | 145 lbs. | 66.0" | Orthodox | 13 | 4 | 1 | |
| Naoya | Ogawa | 6' 4" | 255 lbs. | -- | Orthodox | 7 | 2 | 0 | ||
| Trey | Ogden | Shamurai | 5' 11" | 155 lbs. | 72.0" | Orthodox | 18 | 7 | 0 | |
| Andy | Ogle | The Little Axe | 5' 8" | 145 lbs. | 69.0" | Orthodox | 9 | 6 | 0 | |
| Ho Taek | Oh | Spider | -- | 155 lbs. | -- | 7 | 2 | 1 | ||
| Michiyoshi | Ohara | 5' 10" | 205 lbs. | -- | Orthodox | 0 | 2 | 0 | ||
| Koji | Oishi | 5' 8" | 170 lbs. | -- | Orthodox | 25 | 10 | 10 | ||
| Takayuki | Okada | Giant Ochiai | 6' 1" | 290 lbs. | -- | Orthodox | 3 | 3 | 2 | |
| Yushin | Okami | Thunder | 6' 2" | 170 lbs. | 75.0" | Southpaw | 34 | 12 | 0 | |
| JJ | Okanovich | The Bosnian Bomber | 5' 11" | 155 lbs. | 76.0" | Orthodox | 7 | 1 | 0 | |
| Bolaji | Oki | The Zulu Warrior | 5' 10" | 155 lbs. | 73.0" | Orthodox | 10 | 3 | 0 | |
| Kazuki | Okubo | 5' 11" | 175 lbs. | -- | Orthodox | 7 | 9 | 0 | ||
| Masakatsu | Okuda | -- | -- | -- | 8 | 5 | 0 | |||
| Aleksei | Oleinik | The Boa Constrictor | 6' 2" | 240 lbs. | 80.0" | Orthodox | 60 | 17 | 1 | |
| Michal | Oleksiejczuk | Hussar | 6' 0" | 185 lbs. | 74.0" | Southpaw | 21 | 9 | 0 | |
| Cezary | Oleksiejczuk | 6' 3" | 185 lbs. | 75.0" | Southpaw | 16 | 3 | 0 | ||
| Rafaello | Oliveira | Tractor | 5' 8" | 155 lbs. | 71.0" | Orthodox | 17 | 8 | 0 | |
| Charles | Oliveira | Do Bronxs | 5' 10" | 155 lbs. | 74.0" | Orthodox | 36 | 11 | 0 | |
| Jorge | Oliveira | 6' 1" | 205 lbs. | -- | 5 | 3 | 1 | |||
| Ednaldo | Oliveira | Lula | 6' 5" | 205 lbs. | -- | Orthodox | 17 | 3 | 1 | |
| Alex | Oliveira | Cowboy | 5' 11" | 170 lbs. | 76.0" | Orthodox | 22 | 12 | 1 | |
| Maria | Oliveira | Spider-Girl | 5' 6" | 115 lbs. | 69.0" | Orthodox | 13 | 7 | 0 | |
| Bruno | Oliveira | 6' 4" | 205 lbs. | 80.0" | Orthodox | 8 | 2 | 0 | ||
| Saimon | Oliveira | 5' 4" | 135 lbs. | 72.0" | Orthodox | 18 | 7 | 0 | ||
| Vinicius | Oliveira | LokDog | 5' 9" | 135 lbs. | 70.0" | Switch | 23 | 3 | 0 | |
| Ravena | Oliveira | Kenoudy | 5' 5" | 115 lbs. | 65.0" | Orthodox | 7 | 3 | 1 | |
| Michael | Oliveira | PQD | 6' 1" | 170 lbs. | 78.0" | Orthodox | 9 | 0 | 0 | |
| Wendell | Oliveira Marques | The Machine | 5' 10" | 170 lbs. | -- | Orthodox | 29 | 10 | 0 | |
| Felipe | Olivieri | 5' 9" | 155 lbs. | -- | Orthodox | 14 | 5 | 0 | ||
| Andy | Ologun | 6' 0" | 154 lbs. | -- | Orthodox | 3 | 1 | 0 | ||
| Shana | Olsen | Rock Solid | 5' 7" | 145 lbs. | -- | 5 | 2 | 0 | ||
| Casey | Olson | The Underdog | 5' 7" | 145 lbs. | -- | Orthodox | 14 | 5 | 0 | |
| Dennis | Olson | 6' 0" | 170 lbs. | -- | 14 | 11 | 0 | |||
| Cameron | Olson | Cap'n Crunch | 6' 1" | 205 lbs. | 75.0" | Orthodox | 7 | 3 | 0 | |
| Gadzhi | Omargadzhiev | 6' 1" | 170 lbs. | 71.0" | Switch | 13 | 2 | 0 | ||
| Alan | Omer | 5' 10" | 145 lbs. | -- | Orthodox | 20 | 5 | 0 | ||
| Daniel | Omielanczuk | 6' 0" | 247 lbs. | 74.0" | Southpaw | 19 | 8 | 1 | ||
| Michihiro | Omigawa | 5' 6" | 145 lbs. | 68.0" | Orthodox | 17 | 14 | 1 | ||
| David | Onama | 5' 11" | 145 lbs. | 74.0" | Orthodox | 14 | 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" | 170 lbs. | 74.0" | Orthodox | 14 | 2 | 1 | ||
| Sam | Oropeza | 6' 2" | 170 lbs. | -- | 13 | 3 | 0 | |||
| Brian | Ortega | T-City | 5' 8" | 145 lbs. | 69.0" | Switch | 16 | 5 | 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 | 13 | 9 | 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 | ||
| Quemuel | Ottoni | 6' 0" | 155 lbs. | 72.0" | Orthodox | 12 | 3 | 0 | ||
| 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 | |||
| Shungo | Oyama | 5' 11" | 185 lbs. | -- | Southpaw | 14 | 19 | 0 | ||
| Ren | Ozaki | 5' 7" | 135 lbs. | 68.0" | Orthodox | 6 | 1 | 2 | ||
| Alptekin | Ozkilic | The Turkish Delight | 5' 5" | 125 lbs. | 65.0" | Orthodox | 9 | 5 | 0 |