| Oakland Athletics | Major League Baseball | Oakland | 1968 | Coliseum RingCentral |
| OBG Roma | — | — | — | — |
| Octavio Espinosa de Ica | Copa Perú | — | 1923 | Estadio José Picasso Peratta |
| Oklahoma City Thunder | National Basketball Association | Kota Oklahoma | 2008 | Climate Pledge Arena, Tacoma Dome, Paycom Center, Kingdome |
| Oleggio Magic Basket | — | Oleggio | — | — |
| Olimpia Basket Matera | — | Matera | 1960 | — |
| Olimpia Basket Pistoia | — | Pistoia | 1965 | — |
| Olimpia Milano | Lega Basket Serie A | Milan | 1936 | Forum Mediolanum |
| Olympia 68 Basketball | — | Reggio di Calabria | — | — |
| Olímpico Peruano | Copa Perú | — | 2 Februari 1945 | — |
| Oratorio Elmas | — | — | — | — |
| Orlando City SC | Liga Utama Sepak Bola Amerika Serikat | Orlando | 2013 | Stadion Exploria, Stadion Camping World |
| Orlando Magic | National Basketball Association | Orlando | 1989 | Amway Arena, Kia Center |
| Orzi Basket | — | Orzinuovi | 1964 | — |
| Ottawa Senators | — | Kanata | 1992 | Canadian Tire Centre, TD Place Arena |
| Ottawa Senators | — | — | 1883 | — |