We don't yet know the oxygen pressure in the bottles before the event. Top pressure is believed to be 1800 psi and a recharge reportably required if 1300 psi or less. Ultimate design strength another unknown yet. Charge pressure could have been anywhere between 1300 and 1800 psi..
More likely to be in the upper region. Because QF 744's fly over high terrain in Asia, the miniumum pressure ex-Australia is 1800psi cockpit (EICAS) indication (strongly enforced by the pilots as it's in their manuals). The pressure is not going to drop 500 psi on a trip to Europe and back under normal circumstances.
When filling the bottles at the remote fill port near the Forward Cargo door, the engineers use the gauge at the remote fill port to assess tank oxygen pressure. Although the cockpit indication is supposed to be a repeater (as it uses the same oxygen tank averaging system) the cockpit indications always read lower than the fill point gauge (+/-100psi difference is allowable), so the engineers have to put extra pressure in the tanks to get 1800psi in the cockpit. You might say that the fill point gauge is underreading, but this gauge reading is always closer to the gauge reading on the regularly calibrated oxygen servicing cart. Normally the fill pressure from the cart is higher still. I have, on occasions, on other operators, seen cockpit indications over 2000psi on warm days, so I can only imagine the pressures in the bottles on very hot days. I think the highest pressure I've seen in the cockpit on QF is in the mid 1900's (ex-Sydney).
Anyway, these pressures are all well below the design limits of the bottle and the pressure relief blow out disc value (2650~3083psi)
