| Middlesex to Olympia Heights (Florida) | 1,079.52 mi (1,737.32 km) |
| Middlesex to Omaha (Nebraska) | 1,123.33 mi (1,807.83 km) |
| Middlesex to On Top of the World Designated Place (Florida) | 906.16 mi (1,458.33 km) |
| Middlesex to Onalaska (Wisconsin) | 883.10 mi (1,421.21 km) |
| Middlesex to Oneida (New York) | 182.94 mi (294.42 km) |
| Middlesex to Oneonta (New York) | 133.20 mi (214.36 km) |
| Middlesex to Ontario (California) | 2,387.23 mi (3,841.88 km) |
| Middlesex to Ontario (Oregon) | 2,159.54 mi (3,475.44 km) |
| Middlesex to Opa-locka (Florida) | 1,066.39 mi (1,716.20 km) |
| Middlesex to Opelika (Alabama) | 812.88 mi (1,308.21 km) |
| Middlesex to Opelousas (Louisiana) | 1,204.10 mi (1,937.81 km) |
| Middlesex to Orange (California) | 2,405.12 mi (3,870.67 km) |
| Middlesex to Orange (Texas) | 1,299.08 mi (2,090.67 km) |
| Middlesex to Orange City (Florida) | 891.01 mi (1,433.94 km) |
| Middlesex to Orangeburg (South Carolina) | 601.85 mi (968.58 km) |
