Don't get confused between PAX oxygen and Flight Crew - the checks the flight crew do prior to the flight is only on the crew oxygen system - each crew position on the flight deck is equipped with an oxygen/smoke mask - what the crew are checking is the flow of oxygen to the mask.
Flight crew oxygen is gaseous and there is a required bottle pressure that is checked before flight and the flow to the mask is checked. The masks are stowed in containers on the flight deck which have a test button that allows checking of the system without deploying the mask.
Most pax oxygen is chemical generators (some a/c have gaseous) and the drop out sytem and generators are checked at routine intervals by maintenance.
