the 737 jurassic has O2 bottles for the passengers, the classics changed that to chemical oxygen generators.
If I saw the pax o2 gauge indicating 0 I would probably descend to FL250 or 240 after confirming that indication is real (if I can).
If I saw the CREW oxy pressure indicating 0 I would try by all means to confirm validity of that indication, for example by checking any CBs and putting the masks on and breathing for a while. if I can breathe I ´d probably descend a few thousand feet just in case and keep going....if I can´t breathe or I got reasons to suspect the indication is real I´d probably try to terminate the flight asap, that means Vmo -5 kt to destination (wx and fuel permitting) or a diversion/turnback. This is because crew O2 is not only a need during a depressurization but also during a smoke situation, so if I got no O2 that ´d be a bad day.
THE 737-500 MEL says the CREW O2 pressure indicator may be inop as long as o2 bottle pressure is checked and above min requirements prior to every takeoff so if a diversion or turnback is made the flight can be continued after applying the appropriate dispatch deviation procedure.
One more thing. I believe the instrument uses DC power, just by looking at what it does when you remove power, so if it´s reading 0, it could have failed. I´d probably reset the CB once or twice too.
REgards,
SW.