Oxygen regulator (1) to deliver breathing gas in a cabin (10) of an aircraft comprising a first inlet (12) for pressurized breathing oxygen, a second inlet (14) for diluter gas, an outlet (16) to provide a breathing mixture to a user, and a regulation device adjusting the concentration of breathing oxygen in the breathing mixture. The regulation device comprises a emergency device (30) that adjusts the concentration of breathing oxygen in the breathing mixture as a function of a variation speed of the cabin altitude.