| Name | Source | Type | |
|---|---|---|---|
| Oans | An ancient humanoid species and the founders of the Green Lantern Corps. | ||
| Babylonian mythology | Humanoid | ||
| Octavia Butler's Xenogenesis series | |||
| Oasian | |||
| Ob'enn | |||
| Star Trek | Humanoid | ||
| Oculons | Ascendancy | Humanoid | |
| Doctor Who | |||
| Ogrons | Doctor Who | Humanoid | |
| Old Ones | Warhammer 40,000 | ||
| Olive | Shape-shifting service droid | ||
| Omicronians / Popplers | Futurama | ||
| Doctor Who | Squid-like Humanoid | ||
| Doctor Who | |||
| Orandoans | 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. | |
| Stargate SG-1 | Humanoid, Ascended to a higher plain of existence in non-corporial form | ||
| 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 | |||
| Ortheans | Mary Gentle's Golden Witchbreed | Humanoid | |
| Ortog | Star Control | ||
| Star Control | |||
| Oscar | Tommy and Oscar | Fun and weird pink alien. | |
| Oscillator | |||
| Osirians | L. Sprague de Camp's Viagens Interplanetarias Series | Telepathic saurians. | |
| Creatures with the appearance of demons that help shape the evolutionary patterns of other races. | |||
| Owa | Star Control 3 |