You cannot unlock Oni.
I believe they are available from the start. I think you have to buy them... otherwise it depends onwhere you get it...
Defeat JADE ONI
Oni
it's wedged inside a sofa and in the nursery room, which you have to unlock with the safe in the piano area
One place that Oni appears in Ao Oni is in the library.
A few 1st tier games based on Ao Oni are Abe Oni, Aka Oni,Akame Oni, and Ao Oni Plus.
It is thought that the Oni is inside the mansion because he was a former resident of Ao Oni.
The Oni in Ao Oni is a creature with light purple skin and large black eyes.
When an Oni starts following you in the game Ao Oni, the only place to hide is in a closet.
An Oni is a type of myth
The main protagonist of Ao Oni is named the same as Ao Oni.
Oni stands for Japanese monster.