I'm not sure how you can patent oxygen therapy.
I don't think that would be possible.
You also can't patent herbal medicine. Oh wait, of course you can.
You take out the active ingredient in the herb and change it ever so slightly so that it still has a similar allbeit concentrated affect and more importantly it is unique, so you can own it. (very important) There is a danger of side-effects though, so you will need a lot of money for getting into bed with the required medical associations, having a large marketing campaign, and a large sales force to "suggest" to GPs what they should administer. (don't forget the lawyers too)
Another large cost which is vital is to make sure that you stifle any competition in the non-patented world which could easily and cheaply spring up because anyone with little start-up cash could do it. Use bogus scientific reports paid for by you. Use the press and especially TV programs (after all everyone believes what they see on the box, there are NO exceptions) to trash these "charlatans". Destroy all credibility.
As a last resort, lobby politicians (with your gigantic funds from your last patented herb) to issue laws which ensure your continued oligarchy. Make sure everything has to be registered and approved of only by your "paid for" institutions. Then make sure that any dose of the non-patented herb over the "no effect" amount is illegal. Do this in the guise of "we are only protecting you from overdose". This works very well and only costs a couple of million pounds (very cheap relatively). The EU are suckers for this. In fact, make sure all justification for your anti-competitve measures is "protection". The public are suckers by and large.
Despite all these costs, the rewards of an oligarchy can be very great.