Methods and systems for increasing oxygen concentration. An example system includes an oxygen valve configured to be coupled to an oxygen source, an oxygen plenum coupled to the valve, and a mixing valve. The mixing valve includes an oxygen inlet coupled to the oxygen plenum, an ambient-air inlet, and an outlet configured to be attached to an inlet of a blower of a ventilator. The system also includes a pressure sensor, coupled to the oxygen plenum, and a control device communicatively coupled to the pressure sensor and the oxygen valve. The control device receives a differential pressure, measured by the pressure sensor, and based on the measured differential pressure, generates a control signal to control the oxygen valve to maintain a target pressure of gas within the oxygen plenum.