The Diary puzzle is a puzzle encountered in the basement section of Ao Oni version 5.2. It is a puzzle involving several math and number equations. The puzzle's contents are randomized at the beginning of each new game.
In the basement of the mansion, there is a table with the equation "6912+2865=1518" is embedded.The equation appears, at first, to be mathematically impossible, but actually serves as a hint and explanation for the upcoming part of the puzzle. If the equation's numbers are reversed back to front and flipped upside-down, then the it will read out as 5982+2169=8151, which is a proper equation and shows that the numbers of the next part of the puzzle must be reversed in order to obtain a correct answer. The puzzle does not serve any practical purpose past this point.
After this, a Red Diary in the basement can be found on a bookshelf, which is the first part of the next puzzle. the Red Diary will contain a number that is randomized at the start of each game, something like XXXX=9521. In a table in another room of the basement, a piece of paper will be found that can be combined with the Diary in order to complete the equation. After this, the equation will read out something like XXXX=9521+2598, but the numbers must be flipped and inverted once again leaving the equation like 8652+1256=XXXX. The answer you get for this will be 9908, but as seen in the previous equation, the answer must be reversed and flipped- giving you the answer of 8066. The code can be entered into a safe which will give you the Annex Key.