| Abbreviation | Definition |
|---|---|
| NSEA | National Standard Examination in Astronomy |
| NSFA | Numerical Standards for Fundamental Astronomy |
| NSS | National Space Society |
| NSUM | Non-Spatial Universal Mechanics |
| NTT | New Technology Telescope |
| NWAG | North West Astronomy Group |
| NYAA | North York Astronomical Association |
| O | Oblivion |
| O | Occult |
| OAD | Office for Astronomy Development |
| OAD | Office of Astronomy for Development |
| OAO | Office for Astronomy Outreach |
| OAOG | Ottawa Valley Astronomy and Observers Group |
| OAOT | Orbiting Astronomical Observatory Technology |
| OASI | Orwell Astronomical Society (Ipswich) |
| OBAFGKM | Oh, Be A Fine Girl, Kiss Me |
| OBD | Orbiting Brown Dwarf |
| OBS | Orbiting Brown-dwarf Star |
| OCA | Observatoire de la Côte d'Azur |
| ODAS | OCA (Observatoire de la Côte d'Azur) - DLR (Deutschen Zentrum für Luft- und Raumfahrt) Asteroid Survey |