Tim', there is a pretty long discussion about O2 here: https://clusterbusters.org/forums/topic/6213-basic-non-busting-information/.
Concentrator isn't good, because there's still room air mixed with the O2 it delivers, and the flow rate on a concentrator is too low.
This video shows you the basics. He is using the fancy mask (the O2ptimask or "ClusterO2 Kit"), but the principle is the same with a standard non-rebreather -- the bag fills and you empty it as you inhale. https://www.youtube.com/watch?v=PtFHRIQN17s&t=617s
In the ER, they probably used a standard oxygen mask, with no bag. That's much less efficient than a non-rebreather. In the first link I gave you, there are some other tips.