A container designed as a refillable cylinder used to hold compressed medical oxygen (O2) under safe conditions at high pressure (e.g., 50 - 200 Bar). It is typically filled with O2 when delivered from the gas supplier and includes a valve stem, an opening/closing valve, and will be graded according to size (capacity) and colour-coded to denote O2 content. The cylinder may be made of steel, aluminium (Al) or other ferrous or non-ferrous materials and must be used together with a pressure regulator in order to release the O2 at the correct working pressure. O2 is used as an essential life support gas, for anaesthesia, and for therapeutic purposes.