O
| Name | Established | Location |
| O'Brien Observatory | 1961 | Marine on St. Croix, Minnesota, USA |
| Observatorio Astronómico Nacional (Colombia) | 1803 | Bogotá, Colombia |
| Observatorio Astronomico Nacional Tonantzintla | 1961 | Tonantzintla, Mexico |
| Observatorio Solar Carl Sagan | 2000 | Hermosillo, Sonora, Mexico |
| Oil Region Astronomical Observatory | Venango County, Pennsylvania, USA | |
| Onan Observatory | 1990 | Norwood Young America, Minnesota, USA |
| Ondřejov Observatory | 1898 | Ondřejov, Czech Republic |
| Onsala Space Observatory | 1949 | Onsala, Sweden |
| Ooty Radio Telescope | 1970 | Ooty, Tamil Nadu, India |
| Orchard Hill Observatory | 1965 | Amherst, Massachusetts |
| Orioloromano Observatory | 2007 | Oriolo Romano, Italy |
| Orion 1 (defunct) | 1971 | Low Earth orbit |
| Orion 2 (defunct) | 1973 | Low Earth orbit |
| Otter Creek Observatory | 1995 | Meade County, Kentucky, USA |
| Oulu Cosmic-Ray Station | Oulu, Finland | |
| Owens Valley Radio Observatory | Bishop, California, USA |
Read more about this topic: List Of Astronomical Observatories