There are two kinds of oni, common or lesser oni and dark or greater oni; they are also referred to as separate entities using the terms “ogre” and “demon” respectively.
Common oni are, appropriately enough, the most numerous type of oni and the kind most often described in folktales. Their physical forms vary but share some features in common: a third vertical eye on the forehead, menacing teeth and claws, and great strength and stature. Common oni are boisterous, capricious and cruel, but are easily tricked and while certainly sinister they are generally not thought of as evil in the way that dark oni are. Sometimes outwitting an oni can give a hero supernatural gifts; most of the time the hero will regret his encounter with the oni even if he does not end up in its cook-pot.
Dark oni are rarer and more terrifying than common oni. They have a greater variety of hideous forms (see Shadowlands Oni, OA), although the third vertical eye remains an identifying feature. The different types of dark oni are associated with different forms of suffering and evil. The presence of dark oni in the mortal world always portends disaster, and dark oni physically corrupt the mortal world wherever they go (see Taint, OA). Dark oni roaming the spirit world are drawn to the places in the mortal world where their embodied ill is prevalent, and it is at these places that they can most easily pass into our world and wreak havoc.