A portable mechanical device designed to mix air and oxygen (O2) for mobile O2 administration during first aid or emergency situations. It is a hand-held compact unit designed to be connected to an O2 gas container and includes an O2/air mixing valve, a filter for the removal of exhaled carbon dioxide (CO2), a bag for exhaled gas collection and recycling (rebreathing functionality), and connectors (e.g., for attachment to a breathing mask). This is a reusable device.