O2 35000ft
We used a system rigged on the bunk with a fast jet regulator chest mounted. Had to go to Farnborough to learn how to use it in their decompression chamber. Only problem was that our flow indicator was in the lines to the seats out of sight of the pilots and they had to be monitored by flight eng. Pre-breathing was one of the issues as we couldn't do it anywhere on the ground at CLake and we effectively pre-breathed at low cabin alt during the climb to height. If I remember correctly we did progressively higher drops but only got to just above 33000ft before the opening shocks on the jumpers was deemed to be too high. With a very low time of useful consciousness at height the transfer of jumpers between O2 systems had to be carefully run and monitored. Only one incident when HATS Alm became disconnected just as he was going to open up and passed out. Luckily he was talking on i/c at the time and we realised what had happened and carried out a rapid descent to breathable air and he was ok.