| Fighter | Record | Last Bout |
|---|---|---|
| Ottman Azaitar | 13-3-0 | 2024-12-14 |
| Oumar Sy | 12-3-0 | 2026-09-05 |
| Ovince Saint Preux | 27-18-0 | 2024-10-05 |
| Ozzy Diaz | 10-4-0 | 2026-09-19 |
| Rachael Ostovich | 4-6-0 | 2020-11-28 |
| Rafaello Oliveira | 17-8-0 | 2014-02-22 |
| Ravena Oliveira | 7-5-1 | 2026-08-08 |
| Saimon Oliveira | 18-7-0 | 2025-10-11 |
| Sean O'Connell | 17-9-0 | 2016-12-09 |
| Sean O'Malley | 20-3-0 (1 NC) | 2026-06-14 |
| TJ O'Brien | 20-8-0 | 2011-08-14 |
| Tito Ortiz | 19-12-1 | 2012-07-07 |
| Trey Ogden | 18-7-0 (1 NC) | 2025-01-11 |
| Vinicius Oliveira | 24-4-0 | 2026-06-20 |
| Volkan Oezdemir | 21-8-0 | 2025-11-22 |
| Wendell Oliveira Marques | 29-10-0 | 2015-05-30 |
| Yushin Okami | 34-12-0 | 2018-12-01 |
| Zak Ottow | 17-7-0 | 2019-03-09 |