Oh goodness Skye! I am sorry you are having issues insurance.
I was wondering about this myself since I went on O2 recently. I looked up how much oxygen concentrators cost and it seems like the cost to purchase is less than what my insurance is paying for a rental, which is beyond making sense to me.
I assume you have spoken to both the O2 company and your insurance? This seems like an issue between the two of them and should not involve you (except for making phone calls to make sure the process is fixed before you get sent to collections). My suggestion is to call both. You may need to call each several times to get a straight story, but that is the only way I know how to get insurance crud settled. Each will point fingers at each other, so just be persistent. It is THEIR contract, THEIR agreement... it SHOULD be between them (but we all know that's not how the health industry works).
What also you could do is talk to the HR department of the company providing your health insurance. They sometimes can help to straighten things out. My HR department doesn't deal with our insurance, rather they hired an insurance broker, and they are more than helpful in getting things straightened out. They usually have contacts at the insurance company of people who can actually get things done.
(For an FYI about something else concerning O2)
I was mostly concerned that the rental would eventually max out my lifetime DME allotment which freaked me out (already done that to one insurance policy and on the road to doing it with my current policy). But I as assured that since O2 is considered "life sustaining" it will not go against my maximum.
Good luck!