| San Marcos to Olympia (Washington) | 1,006.62 mi (1,620.00 km) |
| San Marcos to Olympia Heights (Florida) | 2,262.81 mi (3,641.64 km) |
| San Marcos to Omaha (Nebraska) | 1,286.38 mi (2,070.23 km) |
| San Marcos to On Top of the World Designated Place (Florida) | 2,072.73 mi (3,335.74 km) |
| San Marcos to Onalaska (Wisconsin) | 1,578.88 mi (2,540.96 km) |
| San Marcos to Oneida (New York) | 2,332.79 mi (3,754.26 km) |
| San Marcos to Oneonta (New York) | 2,361.24 mi (3,800.05 km) |
| San Marcos to Ontario (California) | 67.25 mi (108.23 km) |
| San Marcos to Ontario (Oregon) | 752.56 mi (1,211.13 km) |
| San Marcos to Opa-locka (Florida) | 2,263.15 mi (3,642.18 km) |
| San Marcos to Opelika (Alabama) | 1,837.82 mi (2,957.69 km) |
| San Marcos to Opelousas (Louisiana) | 1,480.44 mi (2,382.54 km) |
| San Marcos to Orange (California) | 59.40 mi (95.59 km) |
| San Marcos to Orange (Texas) | 1,390.10 mi (2,237.14 km) |
| San Marcos to Orange City (Florida) | 2,133.90 mi (3,434.17 km) |
