This walkthrough of the Annex Basement is for Version 6 of Ao Oni.
Go left and head into the room below, and push the tree out of the way and pull the switch to turn on the lights. Examine the shelf on the right to get the Book, use the Book to obtain the Bookmark, and exit the room and go into the room on the right. Examine the black block below to reveal a locked hidden door. Above, there will be locked door and a keypad. Use the Screwdriver on the keypad, and then use the Bookmark on it, to reveal a lot of colored dots. Try to open the locked door to reveal colors. From left to right, this will show what colors of dots to reveal the code. So if the door was White, Red, Green, Yellow and the player had 4 white dots, 2 red dots, 9 green dots and 5 yellow dots, the code would be 4295. After putting in the code, the door will unlock.
Walk North and Takeshi will start chasing you. Mika will come out of the room above. Head into the room on your left and quickly enter the closet. Wait for Takeshi and Mika to leave the room, and exit the closet. Look at the rug. A bit of the rug will be distorted. Examine that part to get the Disc. Leave the room and go into the room Mika came from. Open the closet to find Takuro. Takuro will have a short conversation with the player and then give the player Vinegar. Exit the room and go back to the entrance of the Annex basement. Head into the room left and north. Examine the curtain to reveal a bath tub. Examine the bath tub to drain the water. Examining the tub again will give player the Red Key. Open your inventory, and use the Vinegar Takuro gave you. The player will now have the Iron Gate Key. Push the tub south to reveal a stair case. Go down it. Enter the Cell and go North. There will be red, yellow and blue buttons. Use the Disc to find out what buttons to push. The player will have to push the colors on the Disc clockwise. One of the colors will fall and examining it's place will give the player the Metal Stick. Combine the Metal Stick with the Disc. You will receive the Hidden Door Key. Go back to the Hidden Door and use the key. The player will find a ladder. The player must now go back to get Takuro and bring him back to the ladder. The Oni will be eating the ladder. The Oni will then kill Takuro and the player must escape The Oni. Return to where Hiroshi found the ladder to find the Item "Backdoor Key". Take the Backdoor Key and go back to where Hiroshi got the Doll Head in the Annex. Put the light bulb on the headless doll, turn off the lights, and use the Plate Shard on where the Shadow is pointing.
The Annex Basement also exists in version 5. In version 5 there are two puzzles: the coloring puzzle and the doll head puzzle. The three pieces (moon, star and sun) are located here.