| Jake O'Brien (inactive) | Irish | 4-3-0 | United States | Jul 11, 2009 |
| TJ O'Brien (inactive) | The Spider | 0-2-0 | United States | Aug 14, 2011 |
| Sean O'Connell (inactive) | The Real OC | 2-5-0 | United States | Dec 9, 2016 |
| Sean O'Malley | Suga | 12-3-0 (1 NC) | United States | Jun 14, 2026 |
| Chuck O'Neil (inactive) | Cold Steel | 0-1-0 | United States | Jun 4, 2011 |
| Casey O'Neill | King | 7-2-0 | United Kingdom | Sep 19, 2026 |
| Brendan O'Reilly (inactive) | Badger | 1-3-0 | Australia | Dec 3, 2016 |
| Jose Ochoa | Kalzifer | 2-2-0 | Peru | May 9, 2026 |
| Volkan Oezdemir | No Time | 9-7-0 | Switzerland | Nov 22, 2025 |
| Kaan Ofli | Genghis | 3-2-0 | Australia | Jun 27, 2026 |
| Trey Ogden | Shamurai | 3-3-0 (1 NC) | United States | Jan 11, 2025 |
| Andy Ogle (inactive) | The Little Axe | 1-5-0 | United States | Jan 24, 2015 |
| Koji Oishi (inactive) | | 0-2-0 | Japan | Jun 4, 2005 |
| Yushin Okami (inactive) | Thunder | 14-7-0 | Japan | Dec 1, 2018 |
| Bolaji Oki | The Zulu Warrior | 2-3-0 | Belgium | Mar 14, 2026 |
| Aleksei Oleinik (inactive) | The Boa Constrictor | 9-8-0 | Russia | Oct 1, 2022 |
| Cezary Oleksiejczuk | | 1-0-0 | Poland | Dec 13, 2025 |
| Michal Oleksiejczuk | Hussar | 10-8-0 (1 NC) | Poland | Jun 27, 2026 |
| Alex Oliveira (inactive) | Cowboy | 11-10-0 (1 NC) | Brazil | Mar 5, 2022 |
| Charles Oliveira | Do Bronxs | 25-11-0 (1 NC) | Brazil | Mar 7, 2026 |
| Edilberto de Oliveira (inactive) | Crocota | 0-2-0 | Unknown | Sep 19, 2007 |
| Ednaldo Oliveira (inactive) | Lula | 0-2-0 | Brazil | Aug 3, 2013 |
| Jorge de Oliveira (inactive) | Blade | 0-2-0 | Brazil | Mar 21, 2015 |
| Maria Oliveira (inactive) | Spider-Girl | 1-3-0 | Brazil | Jun 10, 2023 |
| Michael Oliveira | PQD | 1-0-0 | Brazil | Aug 1, 2026 |
| Rafaello Oliveira (inactive) | Tractor | 2-6-0 | Brazil | Feb 22, 2014 |
| Ravena Oliveira | Kenoudy | 0-4-0 | Unknown | Aug 8, 2026 |
| Saimon Oliveira | | 0-4-0 | Brazil | Oct 11, 2025 |
| Vinicius Oliveira | LokDog | 5-1-0 | Brazil | Jun 20, 2026 |
| Felipe Olivieri (inactive) | | 0-1-0 | Brazil | Jan 30, 2016 |
| Gadzhi Omargadzhiev (inactive) | | 0-2-0 | Russia | Oct 22, 2022 |
| Alan Omer (inactive) | | 0-2-0 | United States | Jun 20, 2015 |
| Daniel Omielanczuk (inactive) | | 4-5-0 | Poland | Jul 8, 2017 |
| Michihiro Omigawa (inactive) | | 1-6-0 | Japan | Aug 4, 2012 |
| David Onama | | 6-3-0 | Uganda | Nov 1, 2025 |
| Charlie Ontiveros (inactive) | American Bad Boy | 0-3-0 | United States | Aug 13, 2022 |
| Myktybek Orolbai | | 5-2-0 | Kyrgyzstan | Aug 15, 2026 |
| Brian Ortega | T-City | 8-5-0 (1 NC) | United States | Aug 23, 2025 |
| Dustin Ortiz (inactive) | | 8-6-0 | Unknown | Jan 19, 2019 |
| Tito Ortiz (inactive) | The People's Champ | 15-11-1 | United States | Jul 7, 2012 |
| Ode Osbourne | The Jamaican Sensation | 5-7-0 (1 NC) | Jamaica | Feb 21, 2026 |
| Nick Osipczak (inactive) | | 2-3-0 | United Kingdom | Nov 13, 2010 |
| Mahammadali Osmanli | The Ineffable | 0-1-0 | Azerbaijan | Sep 26, 2026 |
| Nissen Osterneck (inactive) | | 0-1-0 | United States | Apr 1, 2009 |
| Rachael Ostovich (inactive) | | 1-3-0 | United States | Nov 28, 2020 |
| Zak Ottow (inactive) | The Barbarian | 4-4-0 | United States | Mar 9, 2019 |
| Alistair Overeem (inactive) | The Demolition Man | 12-8-0 | United Kingdom | Feb 6, 2021 |
| Alptekin Ozkilic (inactive) | The Turkish Delight | 1-3-0 | Türkiye | May 9, 2015 |