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.
The Puzzle[]
In the basement of the mansion, there is a table with the equation "6912+2865=1518" embedded. The equation appears, at first, to be mathematically impossible, but actually serves as a hint for the upcoming part of the puzzle. If the equation's numbers are reversed back to front and flipped upside-down, then it will read "5982+2169=8151", which is a proper equation and shows that the numbers of the next part of the puzzle must be reversed and flipped in order to obtain the correct answer. The puzzle does not serve any practical purpose past this point.
The Red Diary with one of it's randomized contents.
The page.
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 8066. The code can be entered into a safe which will give you the Annex Key.