B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
General Maintenance Action
Aircraft; Equipment / Tooling
During preflight of O2 system both pilots simultaneously tested system and pressure dropped to zero. If done individually pressure drop was 200 PSI. Maintenance contacted and valve was found to be partially closed on O2 bottle. It is unknown how many cycles aircraft flew in this condition; but unlikely it would have been discovered if both 02 masks hadn't been tested simultaneously.
B737-800 First Officer reported crew oxygen system anomaly during preflight.