Like the preceding games, The Room: Old Sins is a variation on escape the room games, in which the player is presented with a highly-detailed dollhouse to examine and find various secrets within it by manipulating objects within it and solving its puzzles. 

Parts of these puzzles involve interacting with a unique material known as the Null, which has the ability to warp reality. Using a special eyepiece, the player can see objects made of Null, with hidden objects or puzzles seen through the Null item. Items can be collected and examined, and in some cases, manipulated to reveal another object or prepare the item to be used in a puzzle.

 If the player becomes stuck, the game provides a hint system, where after a certain amount of time, the player can request a hint; there is no penalty for using these.Unlike previous games, where the players worked their way through a series of rooms with one or more puzzles within each, the dollhouse of Old Sins presents several interconnected puzzles between multiple rooms of the dollhouse. The player may have up to three different rooms as well as the exterior of the dollhouse to solve each puzzle, with new rooms becoming accessible through the course of the game. There are interconnected puzzle elements between these rooms as well; for example, water that is being pumped into the kitchen must be heated to generate steam that is needed to drive equipment in the workshop. Each room contains a special artifact as its ultimate "prize". Once this artifact is collected, the room is overtaken by a malevolent entity that is tied to the Null, making it no longer accessible.