An oxygen recycler, also referred to as a atmospheric recycler, atmosphere recycler or atmosphere processor, was a component of a life support system that was responsible for recycling old atmosphere into breathable oxygen.
Oxygen recyclers were standard equipment aboard 22nd century Earth shuttlepod. (ENT: "Exile") They were also used aboard stasis pods maintained by the Retellians during the 22nd century. (ENT: "Precious Cargo")
NX-class starships were equipped with several atmosphere processors. When the Enterprise NX-01 was thrown back in time, the crew made several improvements to the vessel over the next century, including the doubling of the efficiency of the ship's atmosphere processors. (ENT: "E²")
Upon inspection of the automated Romulan drone-ship, boarded by Charles Tucker III and Malcolm Reed, Tucker was unable to find anything that resembled an atmosphere recycler on board the ship. (ENT: "Babel One")