It depends on when in time you're asking.

oni started out in China as anything mysterious — spirits, ghosts and monsters. The root of the wird oni means "hidden" and the Chinese kanji for it means "ghost".

When that moved to Japan it was broadened out to include: obsure gods, yôkai, and viscious and particularly brutal humans. Very wicked humans could become oni after death, with some even transforming before death.

Later under the influence of Buddhism, they became: bringers of disasters, spreaders of disease, and orgrish attendants in hell charged with executing the will of enma and punishing evil doers.

They are usually protrayed as very tall, heavily muscled, red or blue skinned, wild haired, horned, and possessing formidable fangs. They frequently carry large iron clubs and are wear tiger skin loincloths. oni are are fond of eating livestock and humans, and drinking alcohol.

oni are normally portrayed as male, although there are some female oni called kijo or hannya. A womam consumed with jealousy may become a hannya. One of the most famous hannya appears in The Tale of Genji (genji monogatari) considered the first written novel in Japan.

There are numerous folktales, seasonal rituals, stories, plays, and lately movies, anime and manga with oni as characters.