| Abbreviation | Definition |
|---|---|
| ODI | One Degree Imager |
| ODS | Our Dark Skies |
| ODX | Occular Dynamic eXperiment |
| OEFS | Observing Earth From Space |
| OG | Original Gravity |
| OHD | Observational Hubble-parameter Data |
| OHP | Observatoire de Haute Provence |
| OITL | Orbit In The Life |
| OMC | Orion Molecular Cloud |
| OOTU | Origin Of The Universe |
| ORB | Observatoire Royal de Belgique (Royal Observatory of Belgium) |
| ORIG | Origin |
| OSO | Orbiting Solar Observatory |
| OSS | Office of Space Science (NASA) |
| OTS | Oasa Tamura Sugitani |
| OVLBI | Orbiting Very Large Baseline Interferometer |
| OW | Outer Wall |
| OWL | OverWhelmingly Large telescope |
| OZ | Objective Zenith |
| PACA | Pro Am Collaborative Astronomy |