| Nuova Fiamma Stabia | — | — | — | — |
| Nuova Sebastiani Basket Rieti | — | Rieti | 1946 | — |
| Oakland Athletics | Grandes Ligas de Béisbol | Oakland | 1968 | Oakland Coliseum |
| Oklahoma City Thunder | National Basketball Association | Oklahoma City | 2008 | Climate Pledge Arena, Tacoma Dome, Paycom Center, Kingdome |
| Olimpia Basket Matera | — | Matera | 1960 | — |
| Olimpia Basket Pistoia | — | Pistoya | 1965 | — |
| Olimpia Cagliari | — | Cagliari | 1953 | — |
| Olimpia Milano | Lega Basket Serie A | Milán | 1936 | Forum di Milano |
| Olímpico Peruano | Copa Perú | — | 2 de febrero de 1945 | — |
| Oratorio Elmas | — | — | — | — |
| 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 | — |
| Padres de San Diego | Grandes Ligas de Béisbol | San Diego | 1969 | Petco Park |
| Pallacanestro Ares Palermo | — | Palermo | — | — |
| Pallacanestro Biella | — | Biella | 1994 | BiellaForum |
| Pallacanestro Brindisi | — | Bríndisi | 1945 | PalaPentassuglia |
| Pallacanestro Chieti | — | Chieti | 1989 | — |
| Pallacanestro Don Bosco Livorno | — | — | — | — |
| Pallacanestro Femminile Costone | — | — | — | — |
| Pallacanestro Ferrara 2011 | — | Ferrara | 2011 | — |
| Pallacanestro Forlì 2015 | Serie A2 | Forlì | 2015 | Unieuro Arena |
| Pallacanestro Livorno | — | Livorno | — | — |
| Pallacanestro Lucca | — | Lucca | — | — |
| Pallacanestro Marsala | — | — | — | — |
| Pallacanestro Milano 1958 | — | Milán | 1957 | — |
| Pallacanestro Olimpia La Spezia | — | La Spezia | 1969 | — |
| Pallacanestro Orzinuovi | — | Orzinuovi | 1964 | — |
| Pallacanestro Patti | — | — | — | — |
| Pallacanestro Pavia | — | Pavía | 1943 | — |
| Pallacanestro Pozzuoli | — | — | — | — |
| Pallacanestro Reggiana | Lega Basket Serie A | Reggio Emilia | 1974 | PalaBigi |
| Pallacanestro Ribera | — | — | 1969 | — |
| Pallacanestro Sant'Antimo | — | Sant'Antimo | 1989 | — |
| Pallacanestro Senigallia | — | Senigallia | — | — |
| Pallacanestro Treviso | Lega Basket Serie A | Treviso | 1954 | Palaverde |
| Pallacanestro Trieste | Lega Basket Serie A | Trieste | 1975 | Palatrieste |
| Pallacanestro Varese | Lega Basket Serie A | Varese | 1945 | PalaWhirlpool |
| Pallacanestro Virtus Cassino | — | Frosinone | 2011 | — |
| Palmares Catania | — | Catania | — | — |
| Partenio Avellino | — | — | — | — |
| PB63 Lady | — | — | 1963 | — |
| PCR Messina | — | Mesina | — | — |
| Perugia Basket | — | — | — | — |
| Pescara Basket | Serie C (Italian men's basketball league) | Pescara | — | PalaElettra |
| Petrarca Armistizio Patavium | — | Padua | 1934 | — |
| PF Umbertide | — | Umbertide | 2008 | — |