| Oakland Athletics | Major League Baseball | Oakland | 1968 | O.co Coliseum |
| OBG Roma | — | — | — | — |
| Octavio Espinosa de Ica | Copa Peru | — | 1923 | Estadio José Picasso Peratta |
| Oklahoma City Thunder | National Basketball Association | Oklahoma City | 2008 | Climate Pledge Arena, Tacoma Dome, Paycom Center, Kingdome |
| Olimpia Basket Pistoia | — | Pistoia | 1965 | — |
| Olimpia Cagliari | — | Cagliari | 1953 | — |
| Olimpia Milão | Liga Italiana de Basquetebol | Milão | 1936 | Mediolanum Forum |
| Olímpico Peruano | Copa Peru | — | 2 de fevereiro de 1945 | — |
| Oratorio Elmas | — | — | — | — |
| Orlandina Basket | — | Capo d'Orlando | 1978 | PalaFantozzi |
| Orlando City Soccer Club | Major League Soccer | Orlando | 2013 | Inter&Co Stadium, Citrus Bowl |
| Orlando Magic | National Basketball Association | Orlando | 1989 | Amway Arena, Kia Center |
| Ottawa Senators | — | Kanata | 1992 | Canadian Tire Centre, TD Place Arena |
| Ottawa Senators | — | — | 1883 | — |