Oni are roughly humanoid, usually large but sometimes small, and have faces like men or apes or beasts and sometimes even birds. They more often than not have horns, but these can range from tiny nubs to long, sharp, spiraling arcs like an antelope's, or antlers like a dragon's. It is said that an Oni's powers and strenght comes from his horns - break or cut one off and they become weak until it grows back... if it does.
The female oni may appear like a beautiful woman who turns ferocious in a fit of jealous rage.
Savage and wild in nature, they rarely wear much more than a loincloth - often in tiger skin. Oni ni kanabō - an ogre with a spiked iron club - is an expression for overkill, as such a powerful weapon hardly seems necessary in the hands of such a fearsome beast. But they are often depicted carrying such destructive instruments anyway.
Oni are ferocious and strong. They may enjoy hunting Humans and strong beasts for the sake of the hunt - and to feed - as well as protect households from harmful spirits.
Some villages hold yearly ceremonies to drive away oni, particularly at the beginning of Spring.
While oni can certainly be found torturing sinners in Hell, they also menace humans in this world, lurking in the mountains and populating distant countries, and riding in the clouds as the spirits of wind and thunder. In folktales these ogres are usually malicious, man-eating creatures to be feared and slain by valiant heroes, the oni can also have a protective function. The onigawara tiles found at the end of Japanese roofs are so called because they were originally carved in the form of an ogre's face, ferocious scowls intended to frighten away harmful spirits.