Agreed, if you're using Oxymizers or any kind of reservoir, you need to get the flow correct for that person. Usually the calibrations on the gauges are pretty close, but always on the conservative side (as they should be). So the use of a Pulse Oximeter is really needed to get the flow down to where you have enough, but you're still not just blowing out O2 due to too high of a flow. (Or not blowing out more than necessary for the individual to still have enough O2.)
On demand systems should conserve the most O2 by only providing what you need (or adjusted for) when you take a breath. But a well adjusted flow using an Oxymizer should do a pretty good job too.
And with the CO2 accumulation in the larger reservoirs.... Is that related to masks? My Oxymizers have a little flapper that *supposedly* keeps your exhale from going into the reservoir and thereby blowing out all the built up O2. (Granted, no clue how effective the little flapper really is.)