The bottles are a major maintenance item only. I don't remember any hatch or access to that part of the aeroplane.
Unless I'm going colourblind, the large green bottles on QF are easily accessible from the inside of the cargo via press-stud & zipper secured fabric panels (glassfibre tape may be seen covering the press studs for extra smoke/fire protection). Green indicates breathing oxygen, rather than pure oxygen. Bottles on QF are occasionally removed for inspection, but as a rule, they are left undisturbed. Qantas tops up its oxygen via a fitting just forward of the Forward Cargo door. In North America, regs makes things rather difficult for servicing as the bottles have to be removed to be topped up. They are somewhat larger than scuba tanks and can be quite heavy and awkward to handle in a confined space.
a times online article indicated that the pilots had trouble with their oxygen supply...and that there was a secondary supply
There is no secondary supply other than the portable bottles in the cockpit (one on the bulkhead, one inside a stowage bin with roll-down door).
Even the so-called Boeing spokesperson got things wrong. There are definitely not "dozens of bottles".