Fighters: O
Last names starting with O, including retired fighters.
- Jake O'Brien “Irish”15-4-0Light Heavyweight
- TJ O'Brien “The Spider”0-2-0Lightweight
- Sean O'Connell “The Real OC”21-10-0Light Heavyweight
- Sean O'Malley “Suga”20-3-0 (1 NC)Bantamweight
- Chuck O'Neil “Cold Steel”0-1-0Welterweight
- Casey O'Neill “King”12-2-0Women's Flyweight
- Brendan O'Reilly “Badger”6-3-0 (1 NC)Lightweight
- Jose Ochoa “Kalzifer”2-2-0Flyweight
- Volkan Oezdemir “No Time”21-8-0Light Heavyweight
- Kaan Ofli “Genghis”3-2-0Featherweight
- Trey Ogden “Shamurai”3-3-0 (1 NC)Lightweight
- Andy Ogle “The Little Axe”9-6-0Featherweight
- Koji Oishi0-2-0Welterweight
- Yushin Okami “Thunder”40-15-0Welterweight
- Bolaji Oki “The Zulu Warrior”2-3-0Lightmweight
- Aleksei Oleinik “The Boa Constrictor”62-18-1Heavyweight
- Cezary Oleksiejczuk1-0-0Middleweight
- Michal Oleksiejczuk “Hussar”22-10-0 (1 NC)Middleweight
- Alex Oliveira “Cowboy”26-20-1 (2 NC)Welterweight
- Charles Oliveira “Do Bronxs”37-11-0 (1 NC)Lightweight
- Edilberto de Oliveira “Crocota”0-2-0Welterweight
- Ednaldo Oliveira “Lula”0-2-0Light Heavyweight
- Jorge de Oliveira “Blade”0-2-0Lightweight
- Maria Oliveira “Spider-Girl”1-3-0Women's Strawweight
- Michael Oliveira “PQD”1-0-0Welterweight
- Rafaello Oliveira “Tractor”17-8-0Lightweight
- Ravena Oliveira “Kenoudy”0-4-0Women's Flyweight
- Saimon Oliveira0-4-0Bantamweight
- Vinicius Oliveira “LokDog”24-4-0Featherweight
- Felipe Olivieri0-1-0Lightweight
- Gadzhi Omargadzhiev0-2-0Welterweight
- Alan Omer0-2-0Featherweight
- Daniel Omielanczuk26-14-1 (1 NC)Heavyweight
- Michihiro Omigawa19-15-1Featherweight
- David Onama14-3-0Featherweight
- Charlie Ontiveros “American Bad Boy”0-3-0Lightweight
- Myktybek Orolbai16-3-1Welterweight
- Brian Ortega “T-City”16-5-0 (1 NC)Lightweight
- Dustin Ortiz20-9-0Flyweight
- Tito Ortiz “The People's Champ”21-12-1Light Heavyweight
- Ode Osbourne “The Jamaican Sensation”13-9-0 (2 NC)Flyweight
- Nick Osipczak6-3-0Welterweight
- Mehemmedeli Osmanli “The Ineffable”0-1-0Bantamweight
- Nissen Osterneck0-1-0Middleweight
- Rachael Ostovich4-6-0Women's Flyweight
- Zak Ottow “The Barbarian”20-8-0Welterweight
- Alistair Overeem “The Demolition Man”47-19-0 (1 NC)Heavyweight
- Alptekin Ozkilic “The Turkish Delight”9-5-0Flyweight