Yes, it is now possible to fly if you need medical oxygen therapy, but there are some guides lines you MUST adhere to.
You must first verify that the POC you use is allowed with your particular airlines
You must have documentation from your doctor that you or someone traveling with you has been trained on how to use the machine.
You must advise us on the first call that you are traveling with a POC
You must make sure that your POC has the approved warning statement in red: "The manufacturer of this POC has determined this device conforms to all applicable FAA acceptance criteria for POC carriage and use on board aircraft.” . This must be on the outside of the device
You must make sure you have full battery and an extra one as well, in the case of flight delays.
It is also possible to carry a POC on a flight with no intention of using it during the flight. If this applies to you, you should get a doctors note stating that the use of the POC during a flight is not needed.