A respiratory assistance apparatus (10) comprising: a gases inlet or inlets (20)； a supplemental gas connection inlet (24)； a blower unit configured to generate a pressurised gases stream； a humidification unit (17)； a gases outlet (12) for the heated and humidified gases stream； a flow path for the gases stream through the respiratory assistance apparatus from the gases inlet(s) through the blower unit and humidification unit to the gases outlet； a sensor assembly (60) comprising an ultrasound gas composition sensor system for sensing the oxygen concentration within the gases stream； a controller operatively connected to the sensor assembly and which generates a representative oxygen concentration signal indicative of the oxygen fraction of the binary gas mixture； a valve operable to alter or modify the flow rate of the supply of supplemental oxygen； and wherein the controller is configured to adjust the valve based on the oxygen concentration signal.
