Well...I never thought the same thought is gonna keep me up awake. So here's that thought.
Oni is also a Polish word for them.
And oni, I may be wrong to assume this, is also Japanese for demon or something like that so...
Oni-chan to me just make me think that a character saying "Them-chan" and....
That's a little weird. But then again, it's just my native language being confusing as I'm trying to enjoy either some manga or anime.
That's all from me folks.