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Since your husband has his own oxygen tank, you do have to check with the airline as to their policy about bringing and using your own oxygen. If they allow you to use your own oxygen, then you can ask for their policy to request a refund. The only issue is if you already used their oxygen tank. However you need to see their written policy about the use of oxygen on the flight.
You have to notify the airline about this, since oxygen is flammable, and the airline will have to approve it on the flight and store it safely for you. This should not be a problem since it is a medical necessity and prescribed by the doctor. The airline must be notified, so give them a call to discuss with them.