| 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 | 2 | 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 | 7 | 1 | 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 | 7 | 0 | |
| Kaan | Ofli | Genghis | 5' 7" | 145 lbs. | 66.0" | Orthodox | 12 | 3 | 1 | |
| Naoya | Ogawa | 6' 4" | 255 lbs. | -- | Orthodox | 7 | 2 | 0 | ||
| Trey | Ogden | Shamurai | 5' 11" | 155 lbs. | 72.0" | Orthodox | 18 | 6 | 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 |