| Name | Source | Type | | Oans | DC Comics | Humanoid |
| Oannes | Babylonian mythology | Humanoid |
| Oankali | Octavia Butler's Xenogenesis series |
| Ob'enn | Schlock Mercenary |
| Ocampa | Star Trek | Humanoid |
| Oculons | Ascendancy | Humanoid |
| Ogri | Doctor Who |
| Ogrons | Doctor Who | Humanoid |
| Old Ones | Warhammer 40,000 |
| Omicronians / Popplers | Futurama |
| Ood | Doctor Who | Squid-like Humanoid |
| Optera | Doctor Who |
| Orandoans | DC Comics' Legion of Super-Heroes |
| Orfa | Ascendancy |
| Organians | Star Trek | Energy beings dedicated to stopping all forms of violence. |
| Ori | Stargate SG-1 | Humanoid, Ascended to a higher plain of existence in non-corporial form |
| Orion | Star Trek | Green-skinned humanoids known for the illegal trade their extremely attractive females as slaves. |
| Orion Rogues | Battlelords of the 23rd Century |
| Orions | Master of Orion |
| Orkans | Mork and Mindy |
| Orks | Warhammer 40,000 | Humanoid |
| Orsians | Tracker |
| Ortheans | Mary Gentle's Golden Witchbreed | Humanoid |
| Ortog | Star Control |
| Orz | Star Control |
| Oscar | Tommy and Oscar | Fun and weird pink alien. |
| Osirians | L. Sprague de Camp's Viagens Interplanetarias Series | Telepathic saurians. |
| Outsiders | Larry Niven's Known Space |
| Overlords | Childhood's End by Arthur C. Clarke | Creatures with the appearance of demons that help shape the evolutionary patterns of other races. |
| Owa | Star Control 3 |