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1478 drawing by Theodoros Pelecanos of an Ouroboros, in alchemical tract titled Synosius.
- Obake (Japanese) – Shapeshifting spirits
- Obariyon (Japanese) – Spook which rides piggyback on a human victim and becomes unbearably heavy
- Obayifo (Ashanti) – Vampiric possession spirit
- Obia (West Africa) – Gigantic animal that serves witches
- Oceanid (Greek) – Nymph daughters of Oceanus
- Odei (Basque) – Storm spirit
- Odin (Norse mythology) – King of Asgard
- Odmience (Slavic) – Changeling
- Og (Jewish) – Giant king of the Amorites
- Ogopogo (Canadian) Canadian Lake Monster
- Ogun (Nigeria) – Iron god for the Yoruba people (South Western Nigeria)
- Ogre (Medieval folklore) – Large, grotesque humanoid
- Oiwa (Japanese) – Ghost of a woman with a distorted face who was murdered by her husband
- Ojáncanu (Cantabrian) – Giant cyclops who embodies evil.
- Ōkami (Japanese) – Spirit wolf of powerful
- Okiku (Japanese) – Spirit of a plate-counting servant girl, associated with the "Okiku-Mushi" worm
- Ōkubi (Japanese) – Death spirit
- Okuri-inu (Japanese) – Dog or wolf that follows travelers at night, similar to the Black dog of English folklore
- Ole-Higue (Guyanese) – Vampiric hag who takes the form of a fireball at night
- Oluru, Leopard Warrior ((Tanzanian))-A leopard from Tanzania...
- Ōmukade (Japanese) – Giant, human-eating centipede that lives in the mountains
- Oni (Japanese) – Large, grotesque humanoid demon, usually having red skin and horns
- Onibi (Japanese) – Spectral fire
- Onmoraki (Japanese) – Bird-demon created from the spirits of freshly dead corpses
- Onocentaur (Medieval Bestiaries) – Human-donkey hybrid
- Onoskelis (Greek) – Shapeshifting demon
- Onryō (Japanese) – Vengeful ghost that manifests in a physical rather than a spectral form
- Onza (Aztec and Latin American folklore) – Wild cat, possibly a subspecies of cougar
- Oozlum bird (Unknown origin) – Bird that flies backwards
- Ophiotaurus (Greek) – Bull-serpent hybrid
- Opinicus (Heraldic) – Lion-eagle hybrid, similar to a griffin, but with leonine forelimbs
- Orang Bunian (Malay) – Forest spirit
- Orang Minyak (Malay) – Spectral rapist
- Ördög (Hungarian) – Shapeshifting demon
- Oread (Greek) – Mountain nymph
- Ork (Tyrolean) – Little people and house spirits
- Orobas (European) – Horse-headed, honest oracle classed as a demon
- Orphan Bird (Medieval Bestiaries) – Peacock-eagle-swan-crane hybrid
- Orthrus (Greek) – Two-headed dog
- Osiris (Hellenized) – God of the dead and the judge of the underworld
- Oshun (Nigeria) – God of love and fertility
- Otso (Finnish) – Bear spirit
- Ouroboros (Worldwide) – Mystic serpent/dragon that eats its own tail
- Ovinnik (Slavic) – Malevolent threshing house spirit
- Owlman (Cornish) – Owl-like humanoid
