The Abacus Puzzle is a puzzle present in version 5.2 of Ao Oni. It is often noted for it being difficult for non-Japanese players due to it being a traditional Japanese puzzle. It is accessible after solving the Length Puzzle and obtaining the cipher. Solving it gives the code for the basement door.
Each column has one 5-bead (above the center line) and four 1-beads (below the center line). When the 5-bead is pushed down towards the line, it is worth 5, and each 1-bead pushed up towards the line is worth 1. Each of the 10 possible positions of the beads in each column corresponds to each of the 10 familiar digits (0 1 2 3 4 5 6 7 8 9). Translate the beads to a four-digit code to solve the puzzle.