The Oni room (動物園 dōbutsuen?, lit. "zoo") is an area in Version 3.0 and Version 6.23. It features a number of deformed monsters, similar to the Oni, behind a locked jail cell. With the exception of the Blob Oni and Blockman, none of the monsters introduced in this room chase the player.
The Oni room has become a popular icon of the franchise. In many fangames, a reference to the original Oni room is included, where a large room includes various Onis. Many of the monsters shown here is what fuel the creation of fangames.
- Fan Response
Between Version 3 and Version 6, the Oni room switched buildings and appearance.
In Version 3, the Oni room is actually composed of two separate rooms. It is located at the top of the house, in the attic. The enigmatic, mysterious feel of the room is marked by the strange, angular puzzle and the odd, blue squiggles adorning the floors.
The Blob Oni, sometimes called the Eye Oni or Eye-See Oni, is irregular. It looks like an amorphous mass with many eyes on it. This Oni can chase the player in Version 3, where it is hiding the front door key. However, since it is very slow, it can easily be tricked and outrun.
The Blockman Oni, sometimes just called Blockman, is the only monster seen in the Oni room that can be encountered during Version 6.23. In parts of the old building in Version 6.23, Blockman can actually appear and chase the player. From then on, Blockman can also appear randomly. This monster has an odd attack pattern: Blockman will sometimes freeze for a second and charge towards the player. It can also replace the Oni (during randomly occuring chases) through use of the BLOCKMAN cheat.
The Caterpillar Oni, sometimes called Jeremy, is a slow, sluggish monster. It looks like a caterpillar with its head up and has five eyes on each side of its many-legged body.
The Clock Oni resembles a clock with thorns, as if exploding, or a stalactite. It levitates while staying affixed to its position, one of the thorns stretching and shrinking continuously. It seems to be the Satellite Oni looking forwards.
The Deformed Oni resembles the regular Oni, but the sprite itself is completely warped.
A fan favorite because of its odd, comical appearance. The Encephalo Oni is sometimes also called the Jellyfish Oni, Octopus Oni, Calmar Oni or O-ging Oni. The sprite for Encephalo Oni is crafted from the upper half of the original Oni's face, and it is jellyfish-like.
The Hunchback Oni, sometimes called Scooter, is a normal Oni, though its back is hunched over and it is bearing teeth. It opens the gate of the Oni room when Hiroshi approaches it, then proceeds to chase him.
The Satellite Oni has spike-like protrusions from its head and is capable of flight. It floats back and forth without any other movement.
The Squatto, sometimes called Hulk Oni or Billy Oni, is a large, muscled Oni that can sometimes be seen banging its fists on the ground. In Version 3, this Oni kills Mika by crushing her with its fists in this way. Its rhythmic banging wakes Hiroshi from sleeping in the kid's bedroom. Because of its size, it moves slowly in most cases. It is so far the biggest of the onis.
When you enter this room, you will see several Onis in this room behind a metal gate with the Hunchback Oni awaiting near the gate. When you approach it, the Hunchback Oni will open it and it will follow you, the Blockman Oni will be following behind. The door behind you will suddenly lock, so press and hold the up arrow key. As the Oni is about to touch you, the door unlocks and you exit the room. Now return to the room and you will notice that all the Onis have disappeared. Now you can proceed to approach the frame on the opposite side of the room and obtain a blue piece. 
Many fans find the Oni room to be one of the most mysterious and frightening moments of the game. Unofficial Oni games have expanded past the original game, thus expanding the Oni universe. Some of the Onis seen in the Oni room that don't have an impact on the game have even acquired specific personalities or characteristics among the fan community. Many times, fangames will include homage to the original Oni room by incorporating a room much like it, filled with either the same monsters or monsters unique to that game.
Many times, the Oni room will even occur in the exact same way - the player may either accidentally set all the Onis free, like they do in Version 6.23, or have to obtain a key the Onis are hiding in the cage like in Version 3.