Chuchinchow - Point taken!
jabird - Yes you can get cover for
some pre existing conditions but I don't believe that would cover the cost of the provision of oxygen, as a pre requisite for the journey, that would amount to making a claim before the event, a non starter as far as the insurance world is concerned.
If the oxygen is a known requirement prior to the journey then it is down to the passenger or their supporting health services to provide it, or the airline, at a cost.