I have used an oxygen concentrator for the last 2 hours on an 8-hour flight and found it no help. Sitting in the plane just reading didn't take any effort, but I had found that getting off the plane was always a bit of a trial. So, I went through the airline questionnaire, got the doctor's letter (£15) and tried it, hoping to reduce the effect of breathing oxygen-reduced (thin) air. As I wrote, no help. But the staff on Virgin could not have been more kind, telling me to press the call button if/whenever I wanted to use their oxygen bottle. Ryanair recently pronounced that they had changed their way of treating their customers, so their ground and in-flight service should be the same.
Since that test, I've swallowed my pride and now request wheelchair assistance at check-in and landing. Having someone whisk you through all the necessary stop points, to and from the plane is great. The Boss loves it because she doesn't have to queue anywhere, just follow the nice guys pushing me. I've now had this ground service throughout Europe and the States and can highly recommend it, except for arriving at Heathrow. Without fail, the Heathrow team will keep you waiting, treat you like an unwanted animal and tell you that they are very busy today. But we survive !
After such a negative response, I do want to say, "Try your oxygen bottle!" In retrospect, my test was bound to produce different results from a pressurised air bottle. My concentrator was taking in thinner-than-usual air and condensing it. The result could not have given the same result that I get on the ground. Maybe, I should have increased the concentration from 3 to 4 litres/minute, on the plane. I could try it again, but I do love the wheelchair!
Take care of your mum.