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")