The Ao Oni is the creation of human suffering
Wandering around the house, you find prison cells, and even later a strange altar and weird hospital beds. What if some family had a deformed baby, and they kept the baby hidden from the world, and imprisoned it as it got stronger and stronger. The fact that dead things in the house become Ao Onis as well is because of all the suffering the main Oni went through, which turned it from just a weird defect into a supernatural monster.
The Oni are creations of Tommy Wiseau.
The Oni evolve.
- When your friends change, they turn into a being that looks like the original monster, but with their hair. Takuro keeps his human for a while and only changes when you go into your menu. Other versions of the monsters are deformed beyond recognition of humanity. Now, unless they are demons or perversions of other animals, maybe the longer a human is an Ao Oni, maybe the more their body mutates.
The Ao Oni remember their pasts.
- Maybe the Ao Oni remember who they are. Takuro was normal even after it was obvious he was caught. He also recognized Hiroshi. He may just not had changed yet, but it's possible he knew him and was using that to get close to him. This is a bit of Fridge Horror as the Ao Oni seem to want to eat humans and make them like them. They could share some sort of Hive Mind and be unable to stop themselves from killing their friends or they just no longer care.
The Ao Oni can choose their form.
- This goes with the evolution and the "remembers their pasts" theories above. Maybe the Ao Oni are sapient enough to choose their form. Takuro looked human, but obviously wasn't. Maybe they learn to control their bodies and can eventually turn into other forms like the ones locked up in the basement.