Abstract: A medical ventilator is disclosed for switching and mixing oxygen with air in a driving and oxygen mixing means (2) and delivering the mixture to a gas flow control apparatus (3) for application directly to a patient breathing apparatus (5) or to storage in a bellows reservoir (81) of a continuous positive airway pressure facility (4). The ventilator is equipped with a control module (1) having directly set control adjustments (6-9, 105-112) for determining respiratory rate, inspiration-to-expiration ratio, oxygen concentration and the prescribed minute volume of the oxygen mixture to be delivered to a patient. The adjustment control circuitry which performs the calculations, logic and drive operations for switching and mixing oxygen in plural chambers (14, 18, 29, 30) of module (2), the switching of the mixture through module (3), and sensing fill and refill needs of reservoir (81). Module (4) includes apparatus (83, 84) for exerting a positive pressure on the oxygen in reservoir (81).