Abstract: The pre-compression nitrox in-line blender of the present invention has, at an upstream end, an oxygen diffuser for diffusing into an annular cavity of the diffuser oxygen supplied into an innermost cavity from a selectively adjustable pressure regulator regulating a high pressure oxygen reservoir. The annular cavity of the oxygen diffuser communicates, by an air intake aperture in the diffuser, with ambient atmospheric air, whereby a low relative air pressure within the annular cavity draws the ambient atmospheric air through the air intake into the annular cavity. A downstream end of the annular cavity is mounted or mountable to, so as to communicate in unimpeded gaseous communication with, an upstream end of an in-line turbulent mixer.