The invention relates to an oxygen breathing device that includes an oxygen source, in particular a chemical oxygen generator or an oxygen pressure tank, at least one oxygen mask connected via an oxygen supply line to said oxygen source, and a flow control unit adapted to receive a signal indicating ambient pressure or to detect ambient pressure and to control the flow of oxygen through the oxygen supply line depending on said ambient pressure. In some embodiments, the flow control unit is adapted to provide a low oxygen flow at a first ambient pressure and provide a higher oxygen flow at a second ambient pressure which is lower than said first ambient pressure.