The invention relates to an oxygen breathing device for an aircraft, in particular for oxygen supply to crew member in a cockpit of an aircraft, comprising a filter unit having an inflow port and an outflow port, the filter unit being adapted to extract nitrogen from an air flow flowing from the inflow port to the outflow port through the filter unit, wherein the filter unit is adapted to receive compressed air, preferably, bleed air from a precompressor stage of a turbine driving the aircraft, or directly by a separate electrically driven compressor, via a bleed air supply line connected to the inflow port, a first pressure regulator in the bleed air supply line, a first water separator which is arranged in the bleed air supply line in flow direction of the bleed air before the first pressure regulator and a connection port which can be brought in fluid communication with said outflow port of the filter unit and which is adapted to be coupled to at least one oxygen supply mask via an oxygen supply line. According to the invention, a chemical oxygen generator comprising at least one substance adapted to produce oxygen in a chemical reaction and to provide said oxygen to an outflow port of said oxygen generator is provided. The outflow port of said chemical oxygen generator is connected to an auxiliary oxygen supply line and a control unit is adapted to selectively provide oxygen from the filter unit or the chemical oxygen generator to an oxygen supply mask coupled to the connection port.