A method of providing a breath to a human patient having a patient connection connected by a patient circuit to a ventilator device. The method includes delivering both a bolus of oxygen and breathing gases including air to the patient circuit. The patient circuit conducts the bolus of oxygen and the breathing gases to the patient connection. The breathing gases are delivered before an end of the inspiratory phase of the breath. The bolus may be delivered at or before a beginning of an inspiratory phase of the breath, and the delivery of the bolus may be terminated before the end of the inspiratory phase of the breath. The bolus and breathing gases are delivered to the patient circuit via different ventilator connections that isolate the bolus from the breathing gases before they enter the patient circuit.