Experienced just once and the learning point was 'Look before doing anything.' I was examining on what was to be the last trip of my C-141A exchange tour. I had prepped my mask on one of the 2 regulators at the back of the flight deck. We got airborne and climbed to FL 350 for a long night leg, Travis to Wake direct. As we levelled - bang, vibration, condensation, dust. All the chamber runs came back to me, and I still remember thinking "It's one of those!" I swung my left arm around, felt a mask and started to put it to my face. But something felt odd as the harness wasn't fitting as it should, so I held it in place. Next, I'm wondering why a colleague was trying to prise it loose, and that was quickly followed by the glorious feeling of cold O2 flowing and matters getting back to normal. It turned out that one of the Loadmasters had attached his mask to the other regulator and, apparently unaware of how a demand system works, he'd turned it off - and, by not looking properly, I'd been trying to fit his mask which, incidentally, had a different harness fitting from mine. Useful seconds were lost there.
The Loadmaster had gone downstairs before top of climb and, when the decompression occurred, the flight deck door slammed shut - the crew hatch almost above his head had not been properly closed and the change of airflow as we levelled forced it ajar. He tried to open the door, forced shut by the slight pressure differential, and it would not open. He grabbed a pax mask from the wall and it came away in his hand. Next, he headed for the crew toilet where there was an O2 bottle - and his seconds of useful consciousness ran out about then. He was found on the freight deck floor, fortunately in time for application of O2 to get him back to normal too.
So descend and back to Travis.