Methods and apparatus for administering oxygen therapy, and particularly high-flow oxygen therapy is disclosed herein. A respiratory monitoring system may non-invasively determine greatest or average peak inspiratory flow rate of a patient based on biofeedback response received from the patient. Medical air, oxygen, or a combination of both may be delivered to the patient at a flow rate equal to greater than the determined greatest or average peak inspiratory flow rate of the patient to meet or exceed inspiratory demand of the patient. Fraction of oxygen inspired by the patient may be determined based on the greatest or average peak inspiratory flow rate and may be adjusted through high-flow oxygen therapy meeting inspiratory demand to prevent entrainment of ambient air or through low-flow oxygen therapy by accounting for entrainment of ambient air based on the greatest or average peak inspiratory flow rate to address medical needs of the patient.