bubbers is spot on The check on the first flight of the day can be assumed ok with just residual pressure. I know, i checked just after this incident. From memory not all aircraft had the shut off valve in the flight deck but it should be wire locked open and function checked.
An accident waiting to happen in my book.
Oxygen systems should be checked for pressure and flow before each flight.If the engineers have to top it up now and then ,so be it.