For what it's worth, I explained to the London Embassy that I am on an O-A visa which expires in Aug 20 but that I only use for 90 day visits, for which I buy travel insurance for each trip. It would be pointless for me to buy annual health insurance under such circumstances. They replied, 'you can use 90 days insurance until your O-A visa expired then apply for non-O'. Whether the IO at the airport takes this view remains to be seen.