A breathing gas supply system for supplying breathable gas in an aircraft includes an oxygen enriching apparatus which is operable in at least two modes to provide product gas with varying oxygen concentrations. A first feed line feeds more highly enriched product gas to one or more breathing gas outlets, and a second feed line feeds less highly enriched product gas e.g. to an aircraft cabin for breathing during normal high altitude flight. A control means controls oxygen enrichment apparatus to provide highly enriched product gas to the breathing gas outlet or outlets in the first mode of operation or less highly enriched product gas e.g. to the cabin for normal breathing, and controls a diverter valve. Which directs the product gas to the respective feed line.