Abstract: This disclosure describes systems and methods for oxygen reprocessing of respiratory gases in mechanical ventilation. Embodiments described herein provide methods for oxygen reprocessing of expiratory gases wherein exhaled air is reprocessed using filters and adsorbers and delivered to a patient based on a clinically specified oxygen concentration. Embodiments described herein provide for graphical display of the oxygen concentration of the product gas of oxygen reprocessing. Embodiments described herein also disclose an automated ventilator functionality whereby oxygen concentration may be set by clinicians through a user interface display, or a “Oxygen Reprocessing Input” screen, from a general ventilation input screen. Embodiments described herein also disclose a ventilator system configured to undergo oxygen reprocessing of expiratory gases and to provide the product gas to the patient for inspiration.