Well I had this very situation a few years ago on an A-320. We were in the cruise with quite a way to destination when it was noticed that the crew oxygen bottle quantity was very low. As I understood it the only masks that provide effective protection against smoke are our quick donning masks and of course the one smoke hood on the flightdeck. It was decided between us to divert to a company base en-route to get it seen to by an engineer. The smoke part of this equation was the real concern to me.
Best Regards,
Office Pest