I recently flew on American Airlines from my home in Melbourne, FL to Chicago. It involved changing planes in Charlotte, NC at their hub. When I made my reservations, I spoke with their disability desk and informed them that I would be flying with my portable oxygen concentrator (POC). I wasn't able to find their oxygen form on-line and when I asked about it, the employee told me that it is no longer required; that I did not need to have a doctor's prescription, or a signed airline form.
Luckily, I had a blank form (thank you, Edna) that I filled out and asked my physician to sign. No one asked me for it (as they always had on all my flights before this last time) until the last leg of my flight back home - Charlotte to Melbourne.
The flight attendant was very insistent that it was required - I told her that I did have it, but that according to American's disability desk it's no longer required. She then said well, we are American Eagle - we are not the same. I handed it to her, but also told her that I had flown American Eagle (a subsidiary of American Airlines) from Melbourne to Charlotte a few days earlier and that no one asked for it.
So - the point that I'm making is to have your doctor write a letter stating that you are able to fly, that you use whatever make and model of POC you have, that you use x# of liters, in flight, or at all times, etc. and have that letter with you, just in case......
If I had not had it with me, you might have seen me in the news, being dragged off of the plane!!