Narrative:

A hose leading into the oxygen mask storage box ruptured causing the crew oxygen tank to deplete. At cruise altitude FL380 in oceanic airspace captain and I heard a 'pop' followed by a loud hissing noise coming from his oxygen mask storage box. We immediately determined it was a leak and I confirmed it by watching the oxygen pressure on the status page drop. Captain attempted to find a way to stop the leak; but was unable. The decision was made to deviate north of the track and descend to 9500 ft and proceed to ZZZ (our ETOPS alternate) being that we had not yet reached the etp (still some 330 NM away). Diversion procedures outlined in the international flight folder and in the commercial chart en route chart were followed. On initial contact with commercial radio we declared an emergency based on the fact that we could not wait for a new altitude and advised them of our intentions to divert once below rvsm airspace and were requesting the appropriate clearance and 9000 ft as a final altitude. The clearance of direct clearance to follow was received. On initial contact with dispatch; ZZZ was the decided alternate even though ZZZ1 was briefly discussed. Within 20 mins; we were in contact with dispatch again and discussed deviating to ZZZ1. A concern was made to the fact that we had declared an emergency and ZZZ was the 'nearest suitable alternate.' we further advised that the 'emergency' was no longer in effect because we were at a suitable altitude for our condition. It was then decided that ZZZ1 would be a better diversion airport for passenger and maintenance support. We reestablished contact with commercial radio and obtained a new clearance to ZZZ1. After passing along our new ETA to ZZZ1 and estimated fob at arrival we received a new clearance direct ZZZ1 from commercial radio. We attempted to inform that the 'emergency' was over but I believe it was lost in the multiple communications with commercial radio. I did reiterate this fact with center upon VHF contact and we were reclred to ZZZ1. Following the initial diversion the flight continued without event. All pertinent parties were informed and ready upon our arrival. The deafening roar of escaping oxygen over the middle of the ocean at FL380 was followed by confirmation of the leak on the EICAS status page.

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Original NASA ASRS Text

Title: B757 FLT CREW ON OVERWATER FLT HEARD OXYGEN ESCAPING FROM CAPT'S MASK RECEPTACLE. CREW OXYGEN DEPLETED AND FLT CREW DECLARED EMER TO CLEAR TRACK SYSTEM; DESCENDED BELOW 10000 FT AND DIVERTED.

Narrative: A HOSE LEADING INTO THE OXYGEN MASK STORAGE BOX RUPTURED CAUSING THE CREW OXYGEN TANK TO DEPLETE. AT CRUISE ALT FL380 IN OCEANIC AIRSPACE CAPT AND I HEARD A 'POP' FOLLOWED BY A LOUD HISSING NOISE COMING FROM HIS OXYGEN MASK STORAGE BOX. WE IMMEDIATELY DETERMINED IT WAS A LEAK AND I CONFIRMED IT BY WATCHING THE OXYGEN PRESSURE ON THE STATUS PAGE DROP. CAPT ATTEMPTED TO FIND A WAY TO STOP THE LEAK; BUT WAS UNABLE. THE DECISION WAS MADE TO DEVIATE N OF THE TRACK AND DSND TO 9500 FT AND PROCEED TO ZZZ (OUR ETOPS ALTERNATE) BEING THAT WE HAD NOT YET REACHED THE ETP (STILL SOME 330 NM AWAY). DIVERSION PROCS OUTLINED IN THE INTL FLT FOLDER AND IN THE COMMERCIAL CHART ENRTE CHART WERE FOLLOWED. ON INITIAL CONTACT WITH COMMERCIAL RADIO WE DECLARED AN EMER BASED ON THE FACT THAT WE COULD NOT WAIT FOR A NEW ALT AND ADVISED THEM OF OUR INTENTIONS TO DIVERT ONCE BELOW RVSM AIRSPACE AND WERE REQUESTING THE APPROPRIATE CLRNC AND 9000 FT AS A FINAL ALT. THE CLRNC OF DIRECT CLRNC TO FOLLOW WAS RECEIVED. ON INITIAL CONTACT WITH DISPATCH; ZZZ WAS THE DECIDED ALTERNATE EVEN THOUGH ZZZ1 WAS BRIEFLY DISCUSSED. WITHIN 20 MINS; WE WERE IN CONTACT WITH DISPATCH AGAIN AND DISCUSSED DEVIATING TO ZZZ1. A CONCERN WAS MADE TO THE FACT THAT WE HAD DECLARED AN EMER AND ZZZ WAS THE 'NEAREST SUITABLE ALTERNATE.' WE FURTHER ADVISED THAT THE 'EMER' WAS NO LONGER IN EFFECT BECAUSE WE WERE AT A SUITABLE ALT FOR OUR CONDITION. IT WAS THEN DECIDED THAT ZZZ1 WOULD BE A BETTER DIVERSION ARPT FOR PAX AND MAINT SUPPORT. WE REESTABLISHED CONTACT WITH COMMERCIAL RADIO AND OBTAINED A NEW CLRNC TO ZZZ1. AFTER PASSING ALONG OUR NEW ETA TO ZZZ1 AND ESTIMATED FOB AT ARR WE RECEIVED A NEW CLRNC DIRECT ZZZ1 FROM COMMERCIAL RADIO. WE ATTEMPTED TO INFORM THAT THE 'EMER' WAS OVER BUT I BELIEVE IT WAS LOST IN THE MULTIPLE COMS WITH COMMERCIAL RADIO. I DID REITERATE THIS FACT WITH CTR UPON VHF CONTACT AND WE WERE RECLRED TO ZZZ1. FOLLOWING THE INITIAL DIVERSION THE FLT CONTINUED WITHOUT EVENT. ALL PERTINENT PARTIES WERE INFORMED AND READY UPON OUR ARR. THE DEAFENING ROAR OF ESCAPING OXYGEN OVER THE MIDDLE OF THE OCEAN AT FL380 WAS FOLLOWED BY CONFIRMATION OF THE LEAK ON THE EICAS STATUS PAGE.

Data retrieved from NASA's ASRS site as of January 2009 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.