In the way way olden cave project days (1960s and even into the 80s) people would take big steel industrial O2 cylinders underwater since there were no boosters, personal transfill whips were rare, and they needed the volume - in part because decompression algorithms were at best primitive, so when in doubt do a lot.I have never heard of this being done. It would be very hard to do because the valves are so different, and there would be no reason to do it.
At the end of the project, just return the o2 bottle to the gas supply store. Not entirely sure what they were using for 1st stages, welding regs I suspect.