A device for delivering a breathing gas to an on-demand breathing mask (23) comprises—an Onboard Oxygen Generator (1) comprising an oxygen-rich gas outlet end; —compressor means (7) comprising inlet means (6) and outlet means (12), said inlet means being connected to oxygen-rich gas outlet end of said Onboard Oxygen Generator (1) for receiving said gas, said compressor means being operable to increase the pressure of said gas for delivery from said outlet means to the on-demand breathing mask. The device further comprises a connecting means (17) to connect said compressor outlet means (12) to said compressor inlet means (6) so that oxygen-rich gas in excess at the outlet means is redirected to the inlet means.