| Oakland Buccaneers | — | Oakland | 1976 | — |
| Oakland Clippers | National Professional Soccer League | Oakland | 1966 | Oakland Coliseum |
| Oakland County FC | United Premier Soccer League | — | 2015 | — |
| Oakland Roots SC | USL Championship | Oakland | 2018 | Oakland Coliseum |
| Oakland Stompers | North American Soccer League | Oakland | 1978 | Oakland Coliseum |
| OC Blues Strikers FC | USL League Two | — | 2011 | — |
| Ocean City Nor'easters | USL League Two | Ocean City | 1996 | Carey Stadium |
| Ogden Outlaws | USL League Two | — | 2006 | Bonneville High School |
| Ohio Vortex | Major Arena Soccer League | — | 2009 | — |
| OKC Energy FC | USL Championship | Oklahoma City | 2 July 2013 | Taft Stadium |
| Oklahoma Alliance | Women's Premier Soccer League | — | 2007 | — |
| Oklahoma City FC | North American Soccer League | — | 2013 | — |
| Oklahoma City FC | Women's Premier Soccer League | — | 2007 | — |
| Oklahoma City Spirit | Lone Star Soccer Alliance | — | 1990 | — |
| Oklahoma City Warriors | — | Oklahoma City | 1986 | — |
| Oklahoma Football Club | Women's Premier Soccer League | Oklahoma | — | — |
| Omaha Vipers | Major Arena Soccer League | — | 2010 | Omaha Civic Auditorium |
| One Knoxville SC | USL League One | Knoxville | 2021 | — |
| Orange County Blue Star | A-League, USL League Two | Irvine | 1997 | — |
| Orange County SC | USL Championship | Irvine | 2010 | Championship Soccer Stadium |
| Orlando City SC | USL Championship | — | 2010 | Camping World Stadium, ESPN Wide World of Sports Complex |
| Orlando Lions | — | Orlando | 1985 | — |
| Orlando Pride | National Women's Soccer League | Orlando | 2015 | Inter&Co Stadium |
| Orlando Sharks | — | Orlando | 2007 | Amway Arena |
| Orlando Sundogs | A-League | — | 1997 | Camping World Stadium |
| Oxnard Guerreros FC | National Premier Soccer League | Temecula | 2016 | — |