Screwballburling
That is extremely stupid but it is not the first time I have heard of such instances. Personally I think the Captain has the right to make his own mind upover non airworthiness issues. Low O2 is not a defect so do not write it up, just top it up and get on your way and if any one ever questions you (unlikely) just say you did an SMS evaluation and it was the most safe option. Once safety is mentioned no one will argue.
I was in Australia with a Hawker 700 and the batteries needed a cap check. The large local Hawker Bechcraft facility had the capability, were FAA, Aussie and EASA approved, but the EASA approval only covered 800's. I contacted our maintenance provider in the UK for advise. Their reply was no problem, we will send out two engineers club class to do a quality audit on the facility and provided they pass we can issue a one off approval for them to do the work! I think the bill would have been about £19,000.
I will let you guess how I got around the problem.