In the Touhou world, oni are generally jolly and love to drink, feast, and compete with humans in any kind of contest, from simple physical fighting to drinking games. Long ago, when an oni found a human that they liked, they tempted that human into challenging them in anyway the human desired. Should the human lose, they were abducted. Because normal humans were nowhere near an oni's level of ability, humans resorted to sneaky underhanded tactics in order to beat the oni. Tired of being tricked and killed all the time, the oni swore off humans forever and left for a place beyond human reach, or so the legend says. It was later on revealed in Subterranean Animism that the oni have taken refuge underground.
According to Patchouli Knowledge and Yuugi Hoshiguma, oni are weakened by fried beans.
Books and articles:
- Wikipedia entry on Oni