O

Oceanid: Nymphs of the ocean. 

Odei:  A genie of thunder and the personification of storm clouds.

Ogre: An ogre is a large and hideous humanoid monster often found in folk and fairy tales. While commonly depicted as an unintelligent and clumsy enemy, it is dangerous in that it feeds on its human victims. The idea of the ogre has been used as a method of instilling good behavior in children by suggesting that bad behavior attracted and excited ogres, who would then attack, kinap, or even eat their victims.

Ogopogo: A serpant like creatures that is comanly founded sighted in Lake Okanagan in British Columbia, Canada.

Okuri-inu:  A large dog or wolf that follows travelers at night and kills them.

Oni: Demonic youkai, with horns, three eyes and three claws. Their skin color ranges from red, green or blue. They share similarities to western ogres and demons, such as feeding on the flesh of humans, causeing disaters and outwited by the heroes. 

Onibaba: A demonic hag that feeds on the flesh of human beings. 

Onocentaur: Similar to a centaur, but with the lower halfs of a donkey. 

Onryo: The spirit of woman who has died of a violent death and seek vengence on those who wronged her.

Ophiotaurus: A best with the body of a bull and the tail of a spertant. 

Oreads: earth elemental nymphs, who gaurd the mountians, grottos and ravines

Orang Pendek: A small ape like creature that was sighted in the Indonisan Islands of Sumatra. 

Orang-bunian: Woodland spirits of Malyasian folklore.

Ouroboros: Depecited as a serpenat eating on it's own tail.

 Otoroshi: A huge shaggy haired youkai with a grotesque face. They lurk on top of trio-gates at the entrance of Shinto shrines, and will attack an evil person who tries to enter the shrine.