| Merthyr Tudful to Olympia Heights (Florida) | 4,296.30 mi (6,914.22 km) |
| Merthyr Tudful to Omaha (Nebraska) | 4,163.15 mi (6,699.95 km) |
| Merthyr Tudful to On Top of the World Designated Place (Florida) | 4,209.14 mi (6,773.96 km) |
| Merthyr Tudful to Onalaska (Wisconsin) | 3,859.28 mi (6,210.92 km) |
| Merthyr Tudful to Oneida (New York) | 3,290.84 mi (5,296.09 km) |
| Merthyr Tudful to Oneonta (New York) | 3,292.72 mi (5,299.12 km) |
| Merthyr Tudful to Ontario (California) | 5,303.78 mi (8,535.61 km) |
| Merthyr Tudful to Ontario (Oregon) | 4,728.63 mi (7,609.99 km) |
| Merthyr Tudful to Opa-locka (Florida) | 4,283.87 mi (6,894.22 km) |
| Merthyr Tudful to Opelika (Alabama) | 4,162.36 mi (6,698.66 km) |
| Merthyr Tudful to Opelousas (Louisiana) | 4,542.08 mi (7,309.77 km) |
| Merthyr Tudful to Orange (California) | 5,324.20 mi (8,568.47 km) |
| Merthyr Tudful to Orange (Texas) | 4,630.26 mi (7,451.68 km) |
| Merthyr Tudful to Orange City (Florida) | 4,175.66 mi (6,720.08 km) |
| Merthyr Tudful to Orangeburg (South Carolina) | 3,935.76 mi (6,333.99 km) |
