The invention relates to an oxygen breathing device, in particular for cockpit crew member of an aircraft, comprising a pressurized oxygen source for storing pressurized oxygen, an oxygen mask, adapted to fit to nose and mouth of a person to be supplied with oxygen from the pressurized oxygen source, an oxygen supply line connecting the pressurized oxygen source with the oxygen mask, a pressure regulator/safety valve coupled in the oxygen supply line to selectively open or shut off oxygen flow from the pressurized oxygen source to the oxygen mask and/or to control pressure of said oxygen supplied to the oxygen mask. According to the invention, such oxygen breathing device is improved by having a pneumatic seal adapted to pneumatically seal the pressurized oxygen source in a non-use condition, an opening device adapted to open the pneumatic seal in case of an emergency situation requiring oxygen supply to the person, a change over valve adapted to selectively supply oxygen from the pressurized oxygen source or pressurized air from a pressurized air source to the oxygen mask.