The Compressor is a machine, similar to the Generator and the Table Saw, which can be used to refill the empty Oxygen Tank once it is acquired.

One can be found on Swamp Hideout, on the room where the Protagonist enters the Swamp biome on the very start of Chapter 2.

This compressor is broken when found, and in order to be used, it must be repaired with the Compressor Parts, which can be found on the Junkyard and the Radio Tower.

Trivia

In older versions, another compressor, fully operable, can be found on the Radio Tower.