When taking your POC on a commercial flight, it is always best to contact the airline in advance and find out what requirements they may have so that you aren't delayed at the gate.  It varies, but usually you will want to take a copy of your oxygen prescription and a note from your doctor indicating your need for portable oxygen.  Then some airlines also have their own forms available as well.  Below are just a few examples:

Southwest Airlines POC Form

United Airlines POC Form

Alaska Airlines POC Form

American Airlines POC Form

These are examples and you should check with your specific airline to find out what their requirements are before you travel.