Abstract: Operating room fires are prevented by a safety interlock that disables instrumentalities (such as an electrical cautery device) capable of igniting a fire in the presence of oxygen and a fuel source or combustible material such as skin, hair and/or prep agents containing alcohol. Removal of, e.g., the cautery device is sensed by a sensor to disable an oxygen source supplying oxygen to a patient and set a timer. Electrical activation of the cautery device only is permitted upon timing-out of the timer, which timing out is set for a time when oxygen will have cleared from the patient's immediate surroundings. A further safety feature is provided in a further timing, by the same or another timer, of the time during which the patient has been deprived of oxygen. After a short duration, then, the cautery (or other) device is denied electrical power and oxygen is permitted to flow again to the patient.