In addition to your hand luggage, you can bring oxygen concentrators (CPAP) and similar respiratory devices free of charge.

You don't need to make any special arrangements or get medical clearance to bring them.  To avoid problems during the flight or at security controls, the equipment must have a label indicating its acceptance for use on aircraft by the FAA, or you will need to bring supporting documentation.

These devices cannot use the electrical sockets on aircraft, so you therefore bring a supply of charged batteries to cover at least 150% of the total duration of your journey: the flight time and the time on the ground (waiting at the departure, transit and destination airports, extras for any unexpected delays, etc.).
The extra batteries must be carried in your hand luggage, individually packed and protected, and placed under the seat in front of you on the aircraft. For more information, please see the battery regulations.

For safety reasons, CPAP machines may have to be disconnected during the journey and will therefore only be accepted if this disconnection does not interfere with the sick person's health.