List of fictional alien species: O
| Name | Source | Type |
|---|---|---|
| Oans | DC Comics | An ancient humanoid species and the founders of the Green Lantern Corps. |
| Oannes | Babylonian mythology | Humanoid |
| Oankali | Octavia Butler's Xenogenesis series | |
| Oasian | Roswell New Mexico | |
| Ob'enn | Schlock Mercenary | |
| Ocampa | Star Trek | Humanoid |
| Oculons | Ascendancy | Humanoid |
| Ogri | Doctor Who | |
| Ogrons | Doctor Who | Humanoid |
| Old Ones | Warhammer 40,000 | |
| Olive | Adventure Time: Distant Lands | Shape-shifting service droid |
| Omicronians / Popplers | Futurama | |
| Ood | Doctor Who | Squid-like Humanoid |
| Optera | Doctor Who | |
| Orandoans | DC Comics' Legion of Super-Heroes | A humanoid species from the planet Orando. Legion member Princess Projectra is an Orandoan. |
| Orfa | Ascendancy | |
| Organians | Star Trek | Energy beings dedicated to stopping all forms of violence. |
| Orgalorg | Adventure Time | A malevolent primordial space deity who takes the form of the penguin Gunter. He is from an unnamed slimy planet. |
| 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 | |
| Orishan | Ben 10 | A race of mollusk-like humanoids from the desert planet Kiusana in the Andromeda Galaxy. They can absorb and redirect moisture. |
| 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. |
| Oscillator | Adventure Time: Fionna and Cake | |
| 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 |