There's an oxygen storage system that serves the cockpit for emergencies (pax masks are served by chemical oxygen generators, not tanks). If the cockpit system

There's an oxygen storage system that serves the cockpit for emergencies (pax masks are served by chemical oxygen generators, not tanks). If the cockpit system has a leak, it's possible that the crew would not be able to be supplied oxygen in a depressurization or smoke in cockpit scenario, so there's a preflight check that all is working. If the plane failed this check, that's consistent with it being discovered after pax are boarded. It's quite similar to fixing a leak and recharging the AC on your car. If you'd call that maintenance, you should call this maintenance. One of the primary reasons that aviation is so safe is experiences like this. Aircraft fails a preflight check for something and is AOG until fixed. ("Aircraft On Ground", aka "broken".) There's an entire industry around serving maintenance needs and special handling for AOG situations.