They're red and yellow with scales.
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
Odmience (Slavic) - Changeling
Og (Jewish) - Giant king of the Amorites
Ogun (Nigeria) - God of Iron 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.
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
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
