I flew a couple of models of Turbo Commander. It was SOP to turn off the O2 bottle (in the rear compartment) after each day or night of flying to prevent loss through system leakage. Easy enough to forget to turn back on.
Not implying that this is what occurred.
The pressurisation in a few that I flew was woeful. A combination of tired air cycle machines and issues with the underfloor tubing that delivered the air would often mean a 17,000' cabin at 25,000'. And that was with Hot Air temp on full. It was worse with cold temps selected.