Mark, please stop shouting. I am trying to learn how these systems are designed. Whether medical or aviation, their purpose is to convert a very high and non-constant pressure (in the cylinder) into a constant flow at low pressure (into your cannula or mask). Here is what I've garnered so far, which may or may not be correct, but give me facts not labels:
A "pressure regulator" per se regulates pressure, not flow. *IF* its output pressure is constant, then the flow from there through a given small orifice will be constant, even as the source cylinder pressure decreases as the oxygen is used up. This orifice is the flow regulator, separate from the pressure regulator. It operates at a much lower pressure (20-50 PSI) than the cylinder pressure (1000-2000 PSI).
These two "regulators" may be combined in the same physical unit, or separated by a hose. For use in a glider where you cannot reach (or even look at) the pressure regulator on the cylinder, you would want the flow regulator separated and within reach. A typical pellet type flow meter which has a needle valve to adjust the flow then serves as the flow regulator. The pressure in the hose upstream of that meter needs to be reasonably low (say under 50 PSI). If you close that flow valve, what keeps the hose pressure from building up? That's the job of the pressure regulator.
There is a diagram of a diaphragm type pressure regulator here, making its operation clear:
https://en.wikipedia.org/wiki/Pressure_regulator
My question relates to the high pressure valve that is operated by the diaphragm of the pressure regulator, labeled a "poppet" in that diagram. When "closed", to what extent does it leak?