* Obake (Japanese) - Shapeshifting spirits
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* 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
* Odmience (Slavic) - Changeling
* Og (Jewish) - Giant king of the Amorites
* Ogre (Medieval folklore) - Large, grotesque humanoid
* Oiwa (Japanese) - Ghost of a woman with a distorted face who was murdered by her husband
* Oj'ancanu (Cantabrian) - Giant cyclops who embodies evil.
* Okiku (Japanese) - Spirit of a plate-counting servant girl, associated with the "Okiku-Mushi" worm
* Okubi (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
* Omukade (Japanese) - Giant, human-eating centipede that lives in the mountains
* Oni (Japanese) - Large, grotesque humanoid
* 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
* Onryo (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
* "Ord"og (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
* Otso (Finnish) - Bear spirit
* Ouroboros (Worldwide) - Mystic serpent/dragon that eats its own tail
* Ovinnik (Slavic) - Malevolent threshing house spirit
1478 drawing by Theodoros Pelecanosof an Ouroboros, in alchemical tract titled Synosius".
