| Fighter | Record | Last Bout |
|---|---|---|
| Bolaji Oki | 10-4-0 | 2026-03-14 |
| Brian Ortega | 16-5-0 (1 NC) | 2026-09-19 |
| Casey O'Neill | 11-2-0 | 2026-09-19 |
| Cezary Oleksiejczuk | 17-3-0 | 2025-12-13 |
| Charles Oliveira | 37-11-0 (1 NC) | 2026-03-07 |
| David Onama | 14-3-0 | 2025-11-01 |
| Jose Ochoa | 9-2-0 (1 NC) | 2026-05-09 |
| Kaan Ofli | 15-4-1 | 2026-06-27 |
| Mehemmedeli Osmanli | 12-1-0 | 2026-09-26 |
| Michael Oliveira | 10-0-0 | 2026-08-01 |
| Michal Oleksiejczuk | 22-10-0 (1 NC) | 2026-06-27 |
| Myktybek Orolbai | 16-3-1 | 2026-08-15 |
| Oban Elliott | 12-5-0 | 2026-08-01 |
| Ode Osbourne | 13-9-0 (2 NC) | 2026-02-21 |
| Ollie Schmid | 4-3-0 | 2026-05-02 |
| Otari Tanzilovi | 10-2-0 | 2026-06-20 |
| 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 |
| Ravena Oliveira | 7-5-1 | 2026-08-08 |
| Saimon Oliveira | 18-7-0 | 2025-10-11 |
| Sean O'Malley | 20-3-0 (1 NC) | 2026-06-14 |
| 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 |
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