I can provide some information since I fly a major airline carrier with O2 once or twice per year with my portable O2 concentrator (this is their process):
1. Contact the airline’s accommodation # and explain your situation.
2. Have your physician complete the form available on the airline’s website.
3. Unable to check in online. Instead, the day of your flight check in at the counter and provide the letter and concentrator to the airline’s personnel.
4. I was required to have 9 1/2 hours of O2 for a 5 hour flight.
5. I contacted an O2 company in the city I was going to and arranged a delivery and pickup for a large O2 machine at the place I was staying.
I hope this helps you. Enjoy your travels.

I fly with my portable concentrator and so far had not had any problems with the airlines or Security.

Well you need to contact the airlines first and find out what their procedure is. your oxygen supplier has POC rentals if you dont have your own POC, most are pricey,Mine wanted $350 a week+extra charge for batteries.If you have your own then you just need to check with your Doc and get forms and some airlines require variable amounts for whatever your flight time is. A 10 hour flight, you may need 15-20 hours of O2 .Wheelchair assist is Free in most cases and I set up oxygen to be delivered to my hotel/place I was staying,Find a Oxygen supplier in the area you will be staying,contact them,I had to send a FAX from my doc with my O2 script and a credit card,and had O2 waiting for me. Thats why I bought My Inogen with the 2 big batteries 20 hours of time. Just a hint, if your POC is a Continuous You will need many batteries,Pulse uses much less.

Arrange for portable concentrator and enough batteries for 1.5 times scheduled flight/check-in time. Download medical form from airline's site, have it filled in by your doctor, contact airline's "special needs" section on site/by phone about a week in advance; one airline wanted you to fax a copy of medical form so their doctor could approve your flight; arrange for wheelchair if unable to walk distances required at airport.
Enjoy yourself and breathe easy- NOTE: check travel insurance as I have yet to find one in Canada that covers oxygen user over 70! Read any policy carefully especially exemptions!

I have flown with it but had a back up. I actually only had 7 hrs of oxygen w/me. I had the small carry on portable & it only gets about 3 hrs per battery.
Plus u have to have a window seat. They don't want people getting tangled up in your tubes!