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
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.
	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.
	Mork and Mindy
Orks	Warhammer 40,000	Humanoid
	Tracker
	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