Oxygen breathing device, comprising: - 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 (2, 57) to said oxygen source, - 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 (2, 57) depending on said ambient pressure, wherein the flow control unit is adapted to ∘ provide a low oxygen flow at a first ambient pressure, ∘ provide a higher oxygen flow at a second ambient pressure which is lower than said first ambient pressure and ∘ to increase said oxygen flow in at least two steps, preferably to increase said oxygen flow constantly, characterized by a bypass valve arranged in the oxygen supply line (2, 57) and comprising a first flow path and a second flow path comprising a bypass channel in a parallel flow arrangement to said first flow path, wherein said bypass valve is adapted to ∘ direct the flow of oxygen through said first flow path at an ambient pressure above a predetermined level according to a first flow condition and ∘ direct the flow of oxygen through said second flow path at an ambient pressure below a predetermined level according to a second flow condition, ∘ wherein said first flow path has a smaller flow cross section than said second flow path. - 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 (2, 57) to said oxygen source, - 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 (2, 57) depending on said ambient pressure, wherein the flow control unit is adapted to ∘ provide a low oxygen flow at a first ambient pressure, ∘ provide a higher oxygen flow at a second ambient pressure which is lower than said first ambient pressure and ∘ to increase said oxygen flow in at least two steps, preferably to increase said oxygen flow constantly, characterized by a bypass valve arranged in the oxygen supply line (2, 57) and comprising a first flow path and a second flow path comprising a bypass channel in a parallel flow arrangement to said first flow path, wherein said bypass valve is adapted to ∘ direct the flow of oxygen through said first flow path at an ambient pressure above a predetermined level according to a first flow condition and ∘ direct the flow of oxygen through said second flow path at an ambient pressure below a predetermined level according to a second flow condition, ∘ wherein said first flow path has a smaller flow cross section than said second flow path. ∘ provide a low oxygen flow at a first ambient pressure, ∘ provide a higher oxygen flow at a second ambient pressure which is lower than said first ambient pressure and ∘ to increase said oxygen flow in at least two steps, preferably to increase said oxygen flow constantly, ∘ direct the flow of oxygen through said first flow path at an ambient pressure above a predetermined level according to a first flow condition and ∘ direct the flow of oxygen through said second flow path at an ambient pressure below a predetermined level according to a second flow condition, ∘ wherein said first flow path has a smaller flow cross section than said second flow path.