| Houston to Orbetello | 5,613.41 mi (9,033.91 km) |
| Houston to Oria | 5,962.77 mi (9,596.15 km) |
| Houston to Oristano | 5,590.88 mi (8,997.66 km) |
| Houston to Orta Nova | 5,847.90 mi (9,411.28 km) |
| Houston to Orte | 5,662.77 mi (9,113.35 km) |
| Houston to Ortona | 5,752.17 mi (9,257.22 km) |
| Houston to Orvieto | 5,641.26 mi (9,078.72 km) |
| Houston to Orzinuovi | 5,450.81 mi (8,772.24 km) |
| Houston to Osimo | 5,669.70 mi (9,124.50 km) |
| Houston to Ospitaletto | 5,450.99 mi (8,772.53 km) |
| Houston to Ostuni | 5,951.01 mi (9,577.23 km) |
| Houston to Ottaviano | 5,814.57 mi (9,357.64 km) |
| Houston to Ovada | 5,424.17 mi (8,729.35 km) |
| Houston to Ozieri | 5,584.84 mi (8,987.93 km) |
| Houston to Paceco | 5,841.61 mi (9,401.16 km) |
