The term "oni" is roughly equivalent to the English term "demon" or "ogre", and as such can describe
a great variety of entities. 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 thesecan range from tiny nubs to long, sharp, spiraling arcs like an antelope's, or
antlers like a dragon's. Savage
and wild in nature, they rarely wear much more than a loincloth. 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.

The oni's most famous variation - with horns like an ox and wearing a tiger-skin fundoshi,
may be related to the kimon, the demon gate through which the world's misfortunes are said to come.
The gate is located in the northeast, or ushi-tora direction - Ushi and tora being the signs
of the ox and the tiger.

Otherwise the oni's appearance is probably derived from Chinese demons, imported in depictions of the
Buddhist underworld. Emma-Daiō, the king of Jigoku (Hell) is sometimes thought to have two
assistants, the aka-oni (red ogre) and the ao-oni (blue or green ogre).

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. While 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.

Oni are a key part of the Japanese holiday known as setsubun. This festival marks the start
of spring, and the new year in the old lunar calendar. People in ogre masks are ritually driven away, symbolically
warding off misfortune and evil for the coming year. Long ago oni could be repelled by the stench of
burning sardines and other methods, but today it is most popular to toss soybeans (which oni are
said to hate) and shout "Oni wa soto! Fuku wa uchi!" ("Out with demons! In with happiness!").