The oxygen tank had a hose and he would turn the valve for the oxygen tank and put the hose to a man's head and it would blow a hole through his head and kill the man. He would also put the hose pressed against the door door knob on the keyhole and it would blow it off clean and the door would open.

He was using a captive bolt gun/pistol. This is a device that projects a metal bolt with which to kill animals in slaughter houses. (Wilki it)

it was just using the air to poke out a big spike. not using the air alone to do damage.