| Observatorio remoto de la sociedad astronómica de Nuevo México | New Mexico | — |
| Observatorio Robinson | Orlando | — |
| Observatorio SFA | Texas | — |
| Observatorio Smith | Wisconsin | — |
 Observatorio Sproul | Pensilvania | — |
 Observatorio Van Vleck | Connecticut | — |
 Observatorio Vassar College | Nueva York | — |
 Observatorio Vulcanológico de Alaska | Alaska | — |
 Observatorio Vulcanológico de California | California | — |
 Observatorio Vulcanológico de Cascades | Washington | — |
 Observatorio Vulcanológico de Hawái | Hawaii County | 1.231 m |
| Observatorio Vulcanológico de Yellowstone | Wyoming | — |
 Observatorio W. M. Keck | Hawái | 4.145 m |
 Observatorio Warner y Swasey | Ohio | — |
 Observatorio Washburn | Madison | — |
 Observatorio Yerkes | Williams Bay | 334 m |
| Observatorios Tenagra | Cottage Grove | — |
| Observatory | California | — |
| Observatory of the State University of Missouri | Misuri | — |
| OCA-Anza Observatory | (2061) Anza | — |
| Ohio State University Observatory | Lima | — |
| Ohio University Observatory | Athens | — |
 Ohio Wesleyan University Student Observatory | Delaware | — |
| Oil Region Astronomical Observatory | Pensilvania | — |
| Ojai observatory | California | — |
 Old Naval Observatory | Washington D. C. | — |
 Old University of Alabama Observatory | Alabama | — |
| Olin Observatory | Connecticut | — |
| Oliver Observing Station | California | — |
| Omaha observatory | Nebraska | — |
| Optec Observatory | Míchigan | — |
 Orbiting Astronomical Observatory | — | — |
 Orbiting Astronomical Observatory 2 | — | — |
 Orbiting Astronomical Observatory 3 | — | — |
 Orbiting Astronomical Observatory B | — | — |
| OrbitJet Observatory | Colden | — |
 Orchard Hill Observatory | Massachusetts | — |
| Organ Mesa observatory | Las Cruces | — |
 Orgonon | Rangeley | — |
| Oscar G. Mayer Memorial Building | Condado de San Diego | 1.680 m |
| Otter Creek Observatory | Indiana | — |
 Owens Valley Radio Observatory | Inyo County | 1.222 m |
| OWL-Net observatory, Mt. Lemmon | Observatorio Astronómico del Monte Lemmon | — |
| P2 Observatory | Mayhill | — |
| Pacific Lutheran University Keck Observatory | Parkland | — |
| Pacific Northwest Regional Observatory | — | — |
| Pacific Sky Observatory observatory | Saipan Municipality | — |
| Palisades-Dows Observatory | Iowa | — |