B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 5000
Training / Qualification; Troubleshooting; Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action; General Maintenance Action
Human Factors; Environment - Non Weather Related; Aircraft; Procedure
While performing pre-flight (first flight of the day on an aircraft we did not fly in); I performed oxygen check in accordance to Vol I.  Pressure in the system was approximately 1;500 PSI and the system checked normally.  After the Captain showed; he did his oxygen check and it also checked normal.  IAW a crew briefing bulletin that came out approximately a month ago; we did the oxygen check simultaneously and the pressure dropped to about 300 PSI.  We called Maintenance and told him of the problem.  He went down and checked the oxygen valve and reported to us that the valve was closed or very close to closed.  He opened the valve and our subsequent oxygen check checked normally.The crew briefing bulletin outlines the potential problem with the oxygen valve not being open and the check to make sure that it is.  Had there been a requirement for the crew to use oxygen; there could have been a catastrophic result.  There probably needs to be a one time check of all 737 oxygen systems and Maintenance and Boeing (this was an almost brand new aircraft) notification.
Both B737-800 pilots simultaneously used the oxygen system on preflight; as per a new company procedure; and found very low mask pressure; so a maintenance check found the crew oxygen bottle valve nearly closed.