B737-800 first officer reported crew oxygen system anomaly during preflight.

Attributes
ACN	1681722
Time
Date	201909
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Parked
Flight Plan	IFR
Component
Aircraft Component	Oxygen System/Crew
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical

Narrative:

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.

Original NASA ASRS Text

Title: B737-800 First Officer reported crew oxygen system anomaly during preflight.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.