Narrative:

Prior to flight; crew oxygen system gauge read 800+ psi. Both ca and first officer successfully accomplished oxygen system check in accordance with FM. Flight was normal until 1:45 into flight at FL380 when we planned to take a restroom break. The first officer discovered his oxygen mask had no flow. I checked mine; discovering the same thing. Further inspection revealed the crew oxygen gauge read zero. We checked the QRH; discovering no guidance on this malfunction. Though no current issue existed; we were concerned about a potential cabin altitude problem or rapid depressurization; so we coordinated a descent to lower altitude while we discussed continuing to destination or diversion. A quick review of the fom and FM revealed no guidance on this particular malfunction. We ultimately decided we needed to get to 10;000 feet since we had no emergency back-up source of crew oxygen. We needed 5.9 fuel at destination and FMC said we should land with 6.4 after uploading current winds. The taf for ZZZ1 said we should have 6nm 8 sct 15 broken; which was also the current metar. We coordinated with dispatch; he confirmed we should land with 6.4 and could find no guidance against continuing to original destination. We briefed the fas; but not the passengers (our descent was gradual; it was night with under cast and we thought it would be on social media immediately upon landing. No passenger commented on being at lower altitude during the remainder of flight or during deplaning). We also sent an ACARS message to maintenance control about the problem and made a logbook entry (we did not get a response from maintenance control). We had to maneuver to avoid some low altitude storms around ZZZ2; but the flight was otherwise uneventful through landing. We actually landed with 6.5. Maintenance did not meet us so I called maintenance control to ensure they knew of the problem.

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Original NASA ASRS Text

Title: Captain reports that he and the First Officer noticed the crew oxygen pressure gauge in their B737-800 aircraft read zero at FL380. They had performed a 15 second crew O2 mask check during preflight with the O2 pressure holding steady at 800 psi. A quick review of their QRH; FOM; and FM revealed no guidance existed for that type of malfunction. Maintenance troubleshooting could not determine the cause of oxygen loss. The aircraft was released for service.

Narrative: Prior to flight; Crew Oxygen system gauge read 800+ psi. Both CA and FO successfully accomplished Oxygen System check IAW FM. Flight was normal until 1:45 into flight at FL380 when we planned to take a restroom break. The FO discovered his oxygen mask had no flow. I checked mine; discovering the same thing. Further inspection revealed the Crew Oxygen gauge read Zero. We checked the QRH; discovering no guidance on this malfunction. Though no current issue existed; we were concerned about a potential cabin altitude problem or rapid depressurization; so we coordinated a descent to lower altitude while we discussed continuing to destination or diversion. A quick review of the FOM and FM revealed no guidance on this particular malfunction. We ultimately decided we needed to get to 10;000 feet since we had no emergency back-up source of crew oxygen. We needed 5.9 fuel at destination and FMC said we should land with 6.4 after uploading current winds. The TAF for ZZZ1 said we should have 6nm 8 SCT 15 BKN; which was also the current METAR. We coordinated with Dispatch; he confirmed we should land with 6.4 and could find no guidance against continuing to original destination. We briefed the FAs; but not the passengers (our descent was gradual; it was night with under cast and we thought it would be on social media immediately upon landing. No passenger commented on being at lower altitude during the remainder of flight or during deplaning). We also sent an ACARS message to Maintenance Control about the problem and made a logbook entry (we did not get a response from Maintenance Control). We had to maneuver to avoid some low altitude storms around ZZZ2; but the flight was otherwise uneventful through landing. We actually landed with 6.5. Maintenance did not meet us so I called Maintenance Control to ensure they knew of the problem.

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.