Hi Lindsey,
When Allan and I flew across the country, he used oxygen on the longer leg of the trip, but not the other. We brought his pulse oximeter along, too, and he tested it both on the ground and in the air. His sats were running around 94% on the ground, but fell to about 86% when he was in the air. He tested mine just out of curiosity, and mine were running about 95% (no CF here). So, I'd recommend having it along just in case if you think you might need it, especially on such a long trip!
Different airlines have different rules about oxygen onboard. Some airlines still provide it and won't allow you to bring your own, but it comes at a high cost--usually around $50 to $80 per segment, depending on the airline. The airline that I work for (they actually made me sign papers saying I wouldn't reveal that I work for their company online, so I'll just hint that starts with US and ends with Airways) no longer provides oxygen. You can bring an oxygen compressor, but there are only two models that they allow. I can check on the names of those models if you are flying on US Airways or America West. No liquid or compressed oxygen is allowed, though.
Just make sure you check with the airline, and give them plenty of notice that you'll be using oxygen. Just as a warning, I've worked as an agent for a year and have never checked in anyone who was going to be using oxygen onboard, so you might get a blank stare when you check in. We just don't see it that often. Allan booked the oxygen with plenty of advance notice, yet they were clueless at check-in. It took a while to straighten it all out, and to make arrangements for it to be available on the plane. We airline agents are morons, quite frankly. I'm not proud of it but I just want you to be aware of it.
Have fun in Hawaii--we want pictures!!
Jess