An oxygen supply system for providing an adequate oxygen supply mode in an aircraft, having: a preventive oxygen supply system comprising an oxygen source; a preventive oxygen dispensing unit being connectable to said oxygen source; a cabin altitude detection device; and a preventive oxygen control device; and an emergency oxygen supply system having an emergency oxygen dispensing unit being automatically provided with oxygen in case of using said emergency oxygen dispensing unit; wherein the preventive oxygen control device is configured to provide at least the following modes: a pre-wear mode for providing no oxygen of said oxygen source to said preventive oxygen dispensing unit but normal cabin air; a pre-oxygenation mode for providing a predetermined rate of oxygen of said oxygen source to said preventive oxygen dispensing unit being sufficient for generating a pre-oxygenation condition of the user; and a decompression mode for providing high concentrated oxygen of said oxygen source to said preventive oxygen dispensing unit being sufficient for ensuring a minimum oxygenation of the user; wherein the preventive oxygen control device is configured to automatically switch to the decompression mode in case a predetermined cabin altitude threshold is detected by said cabin altitude detection device; wherein the emergency oxygen supply system is configured to generate and transmit a signal to the preventive oxygen supply system indicating the correct functioning of the emergency oxygen supply system in case the emergency oxygen supply system is manually activated by the user; and wherein the preventive oxygen supply system is configured to shut down automatically after receiving said signal. a preventive oxygen supply system comprising an oxygen source; a preventive oxygen dispensing unit being connectable to said oxygen source; a cabin altitude detection device; and a preventive oxygen control device; and an emergency oxygen supply system having an emergency oxygen dispensing unit being automatically provided with oxygen in case of using said emergency oxygen dispensing unit; wherein the preventive oxygen control device is configured to provide at least the following modes: a pre-wear mode for providing no oxygen of said oxygen source to said preventive oxygen dispensing unit but normal cabin air; a pre-oxygenation mode for providing a predetermined rate of oxygen of said oxygen source to said preventive oxygen dispensing unit being sufficient for generating a pre-oxygenation condition of the user; and a decompression mode for providing high concentrated oxygen of said oxygen source to said preventive oxygen dispensing unit being sufficient for ensuring a minimum oxygenation of the user; wherein the preventive oxygen control device is configured to automatically switch to the decompression mode in case a predetermined cabin altitude threshold is detected by said cabin altitude detection device; wherein the emergency oxygen supply system is configured to generate and transmit a signal to the preventive oxygen supply system indicating the correct functioning of the emergency oxygen supply system in case the emergency oxygen supply system is manually activated by the user; and wherein the preventive oxygen supply system is configured to shut down automatically after receiving said signal. a pre-wear mode for providing no oxygen of said oxygen source to said preventive oxygen dispensing unit but normal cabin air; a pre-oxygenation mode for providing a predetermined rate of oxygen of said oxygen source to said preventive oxygen dispensing unit being sufficient for generating a pre-oxygenation condition of the user; and a decompression mode for providing high concentrated oxygen of said oxygen source to said preventive oxygen dispensing unit being sufficient for ensuring a minimum oxygenation of the user; wherein the preventive oxygen control device is configured to automatically switch to the decompression mode in case a predetermined cabin altitude threshold is detected by said cabin altitude detection device; wherein the emergency oxygen supply system is configured to generate and transmit a signal to the preventive oxygen supply system indicating the correct functioning of the emergency oxygen supply system in case the emergency oxygen supply system is manually activated by the user; and wherein the preventive oxygen supply system is configured to shut down automatically after receiving said signal.