It would not be unusual for the mask that was prescribed to be the wrong kind, e.g., NOT an non-rebreather.  The key i to get the medical O2 prescription---you have that. You did not indicate what type of regulator (lpm) you are working with. I've used the "Optimask" setup off and on for over four years.  I've transitioned to the mouth piece (included) rather than the mask, BUT, yes, unless you have a demand regulator---which almost certainly you do not---O2 flow continuously.  with the 3 liter bag setup on the Optimask, the O2 flows into the bag and waste is minimal but still there can be waste.