| ObaBet closed obabet.bet.br | BPX Bets Sports Group Ltda | Brazil SPA (Ministry of Finance) | 1 |
| Ocean Online Casino betocean.com | Ocean Casino Resort | US state | 1 |
| Oddset DE oddset.de | German state lottery companies (DLTB) | Germany | 1 |
| OddsJam oddsjam.com | Gambling.com Group Limited | — | 1 |
| Odibets odibets.com | Not identified | Kenya | 1 |
| OhMyZino ohmyzino.com | Famagousta B.V. | Curaçao | 1 |
| OK Bingo okbingo.co.uk | run by Jumpman Gaming white label | UKGC, Alderney | 1 |
| OktagonBet oktagonbet.com | OktagonBet | Serbia | 1 |
| OleyBet closed oleybet.bet.br | Lindau Gaming Brasil S.A. | Brazil SPA (Ministry of Finance) | 1 |
| Olimpo olimpo.bet | Nexus Group (Intercorp) - La Tinka | Peru MINCETUR | 1 |
| Olive Casino olivecasino.com | run by Jumpman Gaming white label | UKGC, Alderney | 1 |
| OlyBet olybet.ee | Olympic Entertainment Group | Estonia | 6 |
| OlyBet España olybet.es | OlyBet Group (Olympic Entertainment Group) | Spain | 1 |
| Olympia Casino olympia.casino | Dama N.V. / Novatrix SRL network | Tobique, Anjouan | 1 |
| Olymposwin olymposwin.vip | Buscarar SRL | Anjouan | 1 |
| Olympusbet olympusbet.com | Admiral Entertainment Limited (Anjouan licensee) | Anjouan | 1 |
| OnaBet closed ona.bet.br | Grupo Esportes Gaming Brasil | Brazil SPA (Ministry of Finance) | 1 |
| One Casino onecasino.ro | Viva Games SRL | Romania | 1 |
| OneBet onebet.co.za | Not identified | South Africa Western Cape | 1 |
| OneCasino sold onecasino.com | LCKY Group (formerly Glitnor Group) | Denmark, Netherlands | 4 |
| OneDun onedun.com | FGS Software Solutions SRL | Anjouan | 1 |
| OnePlay Casino oneplaycasino.es | Azahar Games Corp | Spain | 1 |
| OneRush onerush.com | Hylas Limited | Estonia | 1 |
| OneSports NL onesports.nl | LCKY Group (formerly Glitnor Group) | Denmark, Netherlands | 1 |
| OneStepCasino onestepcasino.com | L.C.S Limited | MGA | 1 |
| Onextwo DE onextwo.de | IBA Entertainment (OneXTwo) | Germany | 1 |
| OnLuck onluck.com | Tessera | Anjouan | 1 |
| OnlyBets onlybets.com.au | Not identified | Australia VGCCC (Victoria) | 1 |
| OnlyBets closed onlybets.bet.br | Zona de Jogo Negócios e Participações Ltda | Brazil SPA (Ministry of Finance) | 1 |
| Onwin onwin.com | Galactic Infinity Ltd | Anjouan | 1 |
| Optibet optibet.lv | Entain plc | Latvia | 4 |
| Oranje Palace oranjepalace.nl | Holland Gaming Technology Ltd | Netherlands | 1 |
| Oria oria.be | Golden Palace (Belgium) | Belgium | 1 |
| Oriental Casino orientalcasino.ro | New Gambling Solutions | Romania | 1 |
| OrientXpress orienxpress.com | Equinox Dynamic N.V. | — | 1 |
| Original Slots original-slots.de | Tipico Group | Germany | 1 |
| Original Wetten original-wetten.de | Tipico Group | Germany | 1 |
| Oshi Casino oshi.io | Dama N.V. / Novatrix SRL network | Tobique, Anjouan | 1 |
| Otto Casino ottocasino.se | Kindred Group plc | Sweden | 1 |
| Ovitoons Casino ovitoonscasino.com | Wild Blackfin Limited SRL | Anjouan | 1 |
| Oz Lotteries ozlotteries.com | Jumbo Interactive Limited | — | 1 |