An auxiliary oxygen bottle (11) for an aircraft aircrew breathing system is provided with an operating head (12) having a selector valve (51) for connecting either an oxygen enriched breathable gas inlet passage (54) or an auxiliary oxygen inlet passage (15) to an outlet passage (55) which is adapted for connection to an aircrew breathing demand regulator. A lever assembly (70) is automatically actuated following aircraft cabin decompression for control of a servo-pressure controlled diaphragm valve (40) to effect movement of the selector valve (51) to a position in which auxiliary oxygen is delivered to the outlet passage (55). The lever assembly (70) may be manually operated by an aircrew member for selection of auxiliary oxygen or oxygen-enriched breathable gas but re-selection of oxygen-enriched breathable gas is inhibited following cabin decompression if the aircraft is operating above a predetermined altitude.