The balloonist Hawthorne C. Gray established human altitude records twice in 1927. His ascent in May 1927 went to 42,470 ft (12.94 km) which is an altitude the FAA requires the use of a pressure demand mask today, doesn't it? Because at that altitude a diluter demand mask at ambient pressure would be unable to spare you from hypoxia even if breathing 100% oxygen.
In November 1927 Gray broke his own record by reaching an altitude of more than 43,000 ft where you definitely have to be in higher-than-ambient pressure, don't you? He died in this flight, but his death occured when the balloon crashed because his oxygen supply ran out during descent though still too high, rendering him unconcious.
I'm unsure what kind of oxygen supply Gray used during his flights, but it seems to me he was the first human to use oxygen under positive pressure. All subsequent higher flights (such as those of the Piccards) were either in fully pressurized gondolas/cabins or in pressure suits.