The Annex Basement is an area in both Version 5.2 and Version 6.23 of Ao Oni. It is located directly under the Annex. It is a very dark area, with gray walls and floorboards.

The door that leads to the Annex Basement.

The Annex Basement also exists in version 5.2. The layout of this area in version 5.2 differs significantly from the final version. There are two puzzles: the coloring puzzle and the doll head puzzle. The three pieces (moon, star and sun) are located here.