1. A system configured to control a fraction of inspired oxygen in a pressurized flow of breathable gas delivered to lungs of a subject, the system comprising: a pressure generator configured to generate a pressurized flow of breathable gas for delivery to the lungs of a subject, the pressure generator being configured to selectively control the fraction of inspired oxygen of the subject by adjusting a concentration of oxygen in the pressurized flow of breathable gas; a user interface; one or more processors configured to execute computer program modules, the computer program modules comprising: a blood oxygen module configured to determine a level of an oxygenation metric representing an amount of oxygen present in blood of the subject; an oxygen fraction module configured to dynamically determine a therapeutic fraction of inspired oxygen for the subject in accordance with a therapy regimen, wherein the oxygen fraction module is configured to dynamically determine the therapeutic fraction of inspired oxygen needed to maintain a therapeutically beneficial level of oxygenation (i) in accordance with the therapy regimen or (ii) to wean the subject from therapy (iii) based on the determined level of the oxygenation metric in a feedback manner; an interface module configured to receive oxygen fraction constraints from a user via the user interface, wherein the oxygen fraction module is further configured such that the oxygen fraction constraints received from the user via the user interface constrain the dynamic determination of the therapeutic fraction of inspired oxygen; and a control module configured to control the pressure generator to adjust the concentration of oxygen in the pressurized flow of breathable gas to automatically and dynamically change the fraction of inspired oxygen of the subject in accordance with the dynamically determined therapeutic fraction of inspired oxygen constrained by received oxygen fraction constraints.