| Fighter | Record | Last Bout |
|---|---|---|
| Alan Omer | 20-5-0 | 2015-06-20 |
| Aleksei Oleinik | 60-17-1 | 2022-10-01 |
| Alex Oliveira | 22-12-1 (2 NC) | 2022-03-05 |
| Alistair Overeem | 47-19-0 (1 NC) | 2021-02-06 |
| Alptekin Ozkilic | 9-5-0 | 2015-05-09 |
| Andy Ogle | 9-6-0 | 2015-01-24 |
| Bolaji Oki | 10-4-0 | 2026-03-14 |
| Brendan O'Reilly | 6-3-0 (1 NC) | 2016-12-03 |
| 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 |
| Charlie Ontiveros | 11-9-0 | 2022-08-13 |
| Chuck O'Neil | 17-9-0 | 2011-06-04 |
| Daniel Omielanczuk | 19-8-1 (1 NC) | 2017-07-08 |
| David Onama | 14-3-0 | 2025-11-01 |
| Dustin Ortiz | 19-8-0 | 2019-01-19 |
| Edilberto de Oliveira | 29-15-2 | 2007-09-19 |
| Ednaldo Oliveira | 17-3-1 (1 NC) | 2013-08-03 |
| Felipe Olivieri | 14-5-0 (1 NC) | 2016-01-30 |
| Gadzhi Omargadzhiev | 13-2-0 | 2022-10-22 |
| Jake O'Brien | 15-4-0 | 2009-07-11 |
| Jon Olav Einemo | 6-3-0 | 2012-01-28 |
| Jorge de Oliveira | 9-2-0 | 2015-03-21 |
| Jose Ochoa | 9-2-0 (1 NC) | 2026-05-09 |
| Kaan Ofli | 15-4-1 | 2026-06-27 |
| Koji Oishi | 25-10-10 | 2005-06-04 |
| Maria Oliveira | 13-7-0 | 2023-06-10 |
| 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 |
| Michihiro Omigawa | 17-14-1 | 2012-08-04 |
| Myktybek Orolbai | 16-3-1 | 2026-08-15 |
| Nick Osipczak | 6-3-0 | 2010-11-13 |
| Nissen Osterneck | 12-4-0 | 2009-04-01 |
| Oban Elliott | 12-5-0 | 2026-08-01 |
| Ode Osbourne | 13-9-0 (2 NC) | 2026-02-21 |
| Oleg Taktarov | 17-5-2 | 1995-12-16 |
| Oli Thompson | 17-10-0 | 2012-08-04 |
| Oliver Enkamp | 7-2-0 | 2018-03-17 |
| Olivier Aubin-Mercier | 11-5-0 | 2019-07-27 |
| Ollie Schmid | 4-3-0 | 2026-05-02 |
| Oluwale Bamgbose | 6-4-0 | 2017-12-16 |
| Omar Morales | 11-4-0 | 2023-03-18 |
| Omari Akhmedov | 21-6-1 | 2021-07-10 |
| Onassis Parungao | 2-1-0 | 1995-09-08 |
| Orion Cosce | 8-2-0 | 2023-05-20 |
| Orlando Wiet | 1-5-0 | 1994-03-11 |
| Oskar Piechota | 11-3-1 (1 NC) | 2020-06-20 |
| Otari Tanzilovi | 10-2-0 | 2026-06-20 |