Most major carriers now allow the use of "Portable Oxygen Concentrators". You can carry on your own concentrator as well as use during a layover. Inquire with the carrier as to their requirements or if they require an MD letter. Also check with your home oxygen vendor to see if they can provide you with a small portable concentrator. For long trips, battery use may become a factor as they may only last a few hours. Most carriers still offer "in-flight" oxygen tanks but will not provide oxygen therapy during your layover. This is where the use of your own equipment would be beneficial.