| Harpenden to Orbetello | 843.75 mi (1,357.88 km) |
| Harpenden to Oria | 1,157.13 mi (1,862.22 km) |
| Harpenden to Oristano | 927.13 mi (1,492.07 km) |
| Harpenden to Orta Nova | 1,047.86 mi (1,686.37 km) |
| Harpenden to Orte | 878.67 mi (1,414.08 km) |
| Harpenden to Ortona | 950.42 mi (1,529.55 km) |
| Harpenden to Orvieto | 855.98 mi (1,377.57 km) |
| Harpenden to Orzinuovi | 644.94 mi (1,037.94 km) |
| Harpenden to Osimo | 861.29 mi (1,386.12 km) |
| Harpenden to Ospitaletto | 641.80 mi (1,032.88 km) |
| Harpenden to Ostuni | 1,143.35 mi (1,840.05 km) |
| Harpenden to Ottaviano | 1,032.82 mi (1,662.16 km) |
| Harpenden to Ovada | 645.20 mi (1,038.34 km) |
| Harpenden to Ozieri | 894.16 mi (1,439.01 km) |
| Harpenden to Paceco | 1,141.85 mi (1,837.63 km) |
