"I do not know but local TV Helo pilot says they are equiped with a 140 gallon plastic fuel tank right under the main gear box rather than a bladder."
The Astar fuel tank is a 'plastic' cylinder in the central truss box upon which all the airframe is built. Baggage compartments either side and aft of that box, and the bulkhead between the cabin and other areas is the forward face of the 'box'. O2 is commonly installed in one of the baggage bays and will directly abut the 'box' containing the fuel cell.
The main gear box is attached to the top of the central 'truss box' over the tank.
I am unfamiliar with this accident aircraft configuration, but many O2 installations have a separate pilot controlled master cutoff valve installed at the LOX tank. I respectfully disagree with "Homonculus', O2 leaks can occur, but I believe they are infrequent. The only time I've used the O2 cutoff was after a hot patient unloading with O's left on.
I would be interested in the cabin heat and demist system in situations similar to that described this event.