| Oakland Buccaneers | — | Oakland | 1976 | — |
| Oakland Clippers | National Professional Soccer League | Oakland | 1966 | Oakland Coliseum |
| 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 | — |
| Ocala Stampede | USL League Two | Ocala | 2012 | — |
| Ocean City Nor'easters | USL League Two | Ocean City | 1996 | Carey Stadium |
| Ogden City SC | USL League Two | Ogden | — | — |
| Ohio Vortex | Major Arena Soccer League | — | 2009 | — |
| OKC Energy FC | USL Championship | Oklahoma City | July 2, 2013 | Taft Stadium |
| Oklahoma Alliance | Women's Premier Soccer League | — | 2007 | — |
| Oklahoma City FC | Women's Premier Soccer League | — | 2007 | — |
| Oklahoma City FC | National Premier Soccer League | — | 2012 | Oklahoma City University |
| Oklahoma City Spirit | Lone Star Soccer Alliance | — | 1990 | — |
| Oklahoma City Warriors | — | Oklahoma City | 1986 | — |
| Oklahoma Football Club | Women's Premier Soccer League | Oklahoma | — | — |
| Omaha Flames | USL League Two | — | 1996 | Ak-Sar-Ben |
| Omaha Vipers | Major Arena Soccer League | — | 2010 | Omaha Civic Auditorium |
| One Knoxville SC | USL League One | Knoxville | 2021 | — |
| Orlando City SC | Major League Soccer | Orlando | 2013 | Inter&Co Stadium |
| Orlando City SC | USL Championship | — | 2010 | Camping World Stadium, ESPN Wide World of Sports Complex |
| Orlando City U-23 | USL League Two | Lake Mary | 1998 | Seminole Soccer Complex |
| Orlando Lions | — | Orlando | 1985 | — |
| Orlando Pride | National Women's Soccer League | Orlando | 2015 | Inter&Co Stadium |
| Orlando Sharks | — | Orlando | 2007 | Amway Arena |
| OVF Alliance | USL League Two | Houston, Albany | — | — |