hi! Sue has a sticky on the disability board section that explains flying with o2.
I have traveled to wdw with o2 and in order to fly she will need an approved POC(personal oxygen concentrator), she will have to have 3 back up batteries to fly(depends on the lpm and how far you are traveling). Not sure of when you are traveling, but it will be very humid and that will mean she will need more o2 support and more o2 supplies as her bpm will increase due to the heat and the evaporation of the o2 is she uses gas.
I have Apria, and when I travel, the local branch in Orlando supplies me with the o2 kit, the stationary concentrator, and several tanks * m6 and e* for back up in the parks. They don't charge extra for this and you will have to notify them in advance so they can ready your supplies, I had to let them know 1 month in advance her, but each supplier and branch is different.
Also, please make sure you get a script for you mom from the Dr. for the o2 and have him or her complete the airline required form in order to use the o2 aboard the aircraft. I was able to use the bus with no problem and my husband carried 2 extra tanks in a wheeled back pack for me when we were at the park. I chose to use my tanks at the park instead of my POC since I was able to take the tanks on all the rides except for RNRC due to going upside down. I did get to use it till we boarded and then the CM took the tank and had it waiting for me on the other side when it was over.
I have the little should bag so it is really small and can hold the tank with ease, it is heavy at times but I manage. The last time my husband was detained by security for a few minutes because he went into the other line for a bag check and they saw my 2 o2 bottles and he had to explain why he was in possession of them

, I walked over and when they saw my o2 around my face and my bag with the one in use, they let him go.

(bet he will get behind me the next time).
I got stuck standing up many trips and it was kind of hard holding onto the bar and my o2 during the trip and I was really worried about it hitting someone, but It never did as I was holding on to it and the bus handle grip for dear life

. This one older lady offered me her seat and I declined as she needed it much more than I did. She was traveling with her husband in a wheel chair,so she said at least let me hold your o2 for you and I did. She was a God send for that. The last day I took my rollator and got to board with wheel chair and
ecv so a seat was not a problem that night. I was very excited to get a seat

.Hope this helps, need any more info, feel free to pm me. Have a nice night.
EDIT:
http://www.disboards.com/showthread.php?t=595713