An oxygen supply system providing an adequate oxygen supply mode in an aircraft. A preventive oxygen supply system includes an oxygen source, a preventive oxygen dispensing unit connectable to the oxygen source, a cabin altitude detection device, and a preventive oxygen control device provide at least the following modes: a pre-wear mode providing no oxygen of the oxygen source to the preventive oxygen dispensing unit but normal cabin air; a pre-oxygenation mode providing a predetermined rate of oxygen of the oxygen source to the preventive oxygen dispensing unit sufficient for generating a pre-oxygenation condition of the user; and a decompression mode providing high concentrated oxygen of the oxygen source to the preventive oxygen dispensing unit sufficient for ensuring a minimum oxygenation of the user. The preventive oxygen control device automatically switches to the decompression mode in case a predetermined cabin altitude threshold is detected by the cabin altitude detection device.