| 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 |
| Brendan O'Reilly | 6-3-0 (1 NC) | 2016-12-03 |
| 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 |
| 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 |
| Koji Oishi | 25-10-10 | 2005-06-04 |
| Maria Oliveira | 13-7-0 | 2023-06-10 |
| Michihiro Omigawa | 17-14-1 | 2012-08-04 |
| Nick Osipczak | 6-3-0 | 2010-11-13 |
| Nissen Osterneck | 12-4-0 | 2009-04-01 |
| 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 |
| 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 |
| Rachael Ostovich | 4-6-0 | 2020-11-28 |
| Rafaello Oliveira | 17-8-0 | 2014-02-22 |
| Sean O'Connell | 17-9-0 | 2016-12-09 |
| TJ O'Brien | 20-8-0 | 2011-08-14 |
| Tito Ortiz | 19-12-1 | 2012-07-07 |
| 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 |
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