| Oakland Athletics | Grandes Ligas de Béisbol | Oakland | 1968 | Oakland Coliseum |
| OBG Roma | — | — | — | — |
| Oklahoma City Thunder | National Basketball Association | Oklahoma City | 2008 | Climate Pledge Arena, Tacoma Dome, Paycom Center, Kingdome |
| Oleggio Magic Basket | — | Oleggio | — | — |
| Olimpia Cagliari | — | Cagliari | 1953 | — |
| Olimpia Milano | Lega Basket Serie A | Milán | 1936 | Forum di Milano |
| Olympia 68 Basketball | — | Regio de Calabria | — | — |
| Olímpico Peruano | Copa Perú | — | 2 de febrero de 1945 | — |
| Orlando City Soccer Club | Major League Soccer | Orlando | 2013 | Inter&Co Stadium, Estadio 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 | — |