Abstract: A ventilator delivers breathing gas to a patient via a patient circuit connecting the ventilator and the patient. The ventilator is configured to, upon activation of an oxygen boost function of the ventilator, increase the oxygen concentration of the breathing gas so as to deliver oxygen-enriched breathing gas to the patient. The ventilator includes a control unit configured to determine the volume of a part of the patient circuit conveying breathing gas from the ventilator to the patient, determine a delay in delivery of the oxygen-enriched breathing gas to the patient based on the volume, and cause indication of the delay to an operator of the ventilator.