TRW, under NASA sponsorship, is developing an on-board aircraft oxygen generation system. Oxygen is generated by water electrolysis and carbon dioxide is removed from the rebreather loop by an electrochemical carbon dioxide concentrator. The design objectives are to develop a safe, reliable, compact system which would replace the present LOX system, thereby minimizing the need for ground support facilities and reduce time and effort required for servicing. The only periodic servicing required is to refill a water reservoir between flights. The system, with the rebreather loop, requires only the generation of oxygen at a rate equal to approximately 1.5 times that metabolically consumed by the user. This system is also applicable for use in closed environments such as spacecraft and submarines.This paper describes the oxygen system and its design. Projected sizes and weights for a fully-developed prototype are presented. Other applications are discussed.