| Holyhead to Olympia Heights (Florida) | 4,232.17 mi (6,811.02 km) |
| Holyhead to Omaha (Nebraska) | 4,064.14 mi (6,540.60 km) |
| Holyhead to On Top of the World Designated Place (Florida) | 4,137.94 mi (6,659.38 km) |
| Holyhead to Onalaska (Wisconsin) | 3,761.08 mi (6,052.87 km) |
| Holyhead to Oneida (New York) | 3,206.37 mi (5,160.15 km) |
| Holyhead to Oneonta (New York) | 3,209.82 mi (5,165.70 km) |
| Holyhead to Ontario (California) | 5,195.15 mi (8,360.79 km) |
| Holyhead to Ontario (Oregon) | 4,615.33 mi (7,427.65 km) |
| Holyhead to Opa-locka (Florida) | 4,219.62 mi (6,790.82 km) |
| Holyhead to Opelika (Alabama) | 4,082.89 mi (6,570.78 km) |
| Holyhead to Opelousas (Louisiana) | 4,457.90 mi (7,174.30 km) |
| Holyhead to Orange (California) | 5,215.56 mi (8,393.63 km) |
| Holyhead to Orange (Texas) | 4,544.76 mi (7,314.08 km) |
| Holyhead to Orange City (Florida) | 4,105.94 mi (6,607.87 km) |
| Holyhead to Orangeburg (South Carolina) | 3,860.27 mi (6,212.50 km) |
