| Ohio State University Observatory | Estados Unidos | Lima | — |
| Ohio University Observatory | Estados Unidos | Athens | — |
 Ohio Wesleyan University Student Observatory | Estados Unidos | Delaware | — |
| Oil Region Astronomical Observatory | Estados Unidos | Pensilvania | — |
| Oishi Kogen Observatory | Japón | Aridagawa | — |
| Oishi observatory | Japón | Kimino | — |
| Ojai observatory | Estados Unidos | California | — |
| Ojima Observatory | Japón | Ojima | — |
| Okayama Astrophysical Observatory | Japón | — | — |
| Okutama observatory | Japón | Okutama | — |
 Old Innsbruck Observatory | Austria | Innsbruck | — |
 Old Naval Observatory | Estados Unidos | Washington D. C. | — |
 Old Observatory of Geneva | Suiza | Ginebra | — |
| Old Orchard Observatory | Reino Unido | Fiddington | — |
 Old Perth Observatory | Australia | City of Perth | — |
 Old University of Alabama Observatory | Estados Unidos | Alabama | — |
| Oldenburg observatory | Alemania | Oldemburgo | — |
| Olin Observatory | Estados Unidos | Connecticut | — |
| Olive Farm Observatory | Reino Unido | South Ribble | — |
| Oliver Observing Station | Estados Unidos | California | — |
| Olmen observatory | Bélgica | Hendrick van Balen | — |
| Omaha observatory | Estados Unidos | Nebraska | — |
| OmegaLab observatory | Italia | Palermo | — |
 Omni-Theatre, Science Centre Singapore | Singapur | Jurong East | — |
| Omsk-Yogik Observatory | Rusia | Omsk | — |
 Ondake Observatory | Japón | Nagasaki | — |
| Ondokuz Mayıs University Observatory | Turquía | — | — |
| Ondrejov--BlueEye600 Telescope observatory | República Checa | Jędrzejów | — |
| Onnens Observatory | Suiza | Prez | — |
 Onsala Space Observatory | Suecia | Kungsbacka | 20 m |
| Ontinyent observatory | España | Onteniente | — |
| Oosato observatory | Japón | Kumagaya | — |
| Ootake observatory | Japón | Ōtake | — |
| OPERA Observatory | Francia | Saint-Palais | — |
| Oppolzer Observatory | Austria | Viena | — |
| Optec Observatory | Estados Unidos | Míchigan | — |
| Orbis Tertius observatory | Argentina | Córdoba | — |
 Orbiting Astronomical Observatory | Estados Unidos | — | — |
 Orbiting Astronomical Observatory 2 | Estados Unidos | — | — |
 Orbiting Astronomical Observatory 3 | Estados Unidos | — | — |
 Orbiting Astronomical Observatory B | Estados Unidos | — | — |
| OrbitJet Observatory | Estados Unidos | Colden | — |
| Orcadas Magnetic Observatory | Argentina | — | — |
 Orchard Hill Observatory | Estados Unidos | Massachusetts | — |
 ORFEUS | — | — | — |
 ORFEUS-SPAS I | — | — | — |
| Organ Mesa observatory | Estados Unidos | Las Cruces | — |
 Orgonon | Estados Unidos | Rangeley | — |