There is something that bugs me about the AD&D Ogre Mage. I think it's because there is an assumption by the game that the Eastern and Western conceptions of the ogre are congruent. While the western conception of the orge is that of a brutish humanoid, roughly larger than a man; the eastern idea is more along the lines of a demon or evil spirit. To me, trying to link the two is like fitting a square peg in a round hole (provided that the length of the square's diagonal is greater than the diameter of the circle). So I think I am going to try to create an oni that fits in more with the eastern interpretation.
According to the internet substitute for real research, Wikipedia, the oni was originally an invisible spirit that caused misfortune. Later influences gave the oni it's current form as a monstrous horned club-wielding demon. So what I am looking at is an evil, corrupting spirit. By itself, it is mostly powerless and vulnerable. But it's true power comes out when it finds a host to exploit to it's fullest potential.
I had hope to have the finished monster ready for this post, but I think I will need some time to work out the details. Consider this issue tabled until I get the concept fully fleshed out.