List of fictional alien species: O
| Name | Source | Type |
|---|---|---|
| Oans | DC Comics | Humanoid |
| 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 | |
| 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 red-shelled mollusk-like humanoids from the desert planet Kiusana in the Andromeda Galaxy. They are very important in the society of their arid home planet, as they are powerful hydrokinetics, able to draw in moisture from the air, unleash water blasts from portholes in their hands, and control existing water at will. They are also quite durable in spite of their thin appearance. |
| 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 |