it depends on what airline you're with, but most provide medical O2 at no charge as far as I know. (The medical O2 doesn't require an extra seat, so even if there is a charge, it would be nominal compared to paying for another seat) This is O2 that is either constant flow or a pulse dose 'on demand' valve, and can flow at rate of 15+ litres per minute. (max flow will depend on what brand or type is being used. Some may go up to 25lpm) it varies from the typical set up on the ground (in a hospital or ambulance set up) in that is is officially rated for use in flight, but that is about it.
The emergency O2 setup runs at 2-4 lpm, nothing more. if your Dad has COPD or similar, he will require a higher flow than that as the cabin altitude is typically around 5-7000 ft above sea level for the cabin pressure. If his O2 saturation can drop below 70%, the 4lpm is unlikely to be a huge help
DON'T rely on the aircraft emergency O2 if he has a known condition that could require extended use of O2. The crew can also get a bit squirrely about using the onboard O2 and may want to start pressing panic buttons if it's used. A flight from the UK to AUS will take about 20 hrs or so, depending on your routing. the emergency bottles would last about half that time, depending on what aircraft and how many are available. if you have a stopover, the crew would quite likely offload your Dad for a medical checkup rather than let him continue if he's been on O2 for most of the previous leg.
If you arrive on board with a medical certificate and O2 ready to be used, it will be a much more routine set up.
Landing into AUS, if there is a passenger with a previously undeclared medical condition who's been hogging all the O2 for the last 10 hrs, the quarantine staff will hold the aircraft until all has been checked out, make you fill out a whole stack of paperwork and give you a check up so thorough you'll think you've been kidnapped by martians BEFORE they let anyone at all disembark.
I hope that is a help, if you have any specifics on the flight or condition I can try and answer them for you.
Last edited by gadgetman; 6th Mar 2008 at 11:19.