ACR CREW DISCOVERS CREW OXYGEN IS DEPLETED AT FL340 WHILE FLYING IN NEW YORK OCEANIC AIRSPACE. THEY DSND TO FL250 AND CONTINUED TO MIA.

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AFTER DEP FROM AUA; ABOUT 2.5 HRS INTO FLT; A CHK OF THE CREW OXYGEN SUPPLY PRESSURIZATION REVEALED COMPLETE LOSS. AS PER PROCS; WE (FO AND MYSELF) IMMEDIATELY OFFSET 15 MI W OF AIRWAY WHILE DSNDING TO FL250. NEW YORK RADIO WAS ALSO ADVISED OF OUR SITUATION BY HF RADIO. CLOSEST ARPTS; BEING OVER 400 MI TO SEA; WERE NAS; PLS; FPT. I CHOSE; OF COURSE; TO CONTINUE TO MIA; NOT BEING IN AN EMER CONDITION. AN EMER WAS NOT DECLARED. #1 FLT ATTENDANT WAS QUICK TO BRING 2 WALKAROUND BOTTLES FOR US UP FRONT (NOT USED). ZNY CLRED US DIRECT TO MIA VIA HF. FO WAS ALSO VERY BUSY WITH DISPATCH WITH SITUATION; DIVERT; CREW LEGALITY; MAINT AND FUEL REQUIREMENTS; ETC. AFTER DOCKING AT GATE; MIA MAINT WAS ON THE ACFT RIGHT AWAY; FOUND THAT A PREVIOUS JUMPSEAT RIDER HAD IMPROPERLY STOWED A MASK; THAT AT SOME POINT INFLT BLED THE TANK DRY. TANK WAS REPLACED AND LEAK TESTED. HERE'S MORE: IRS DEV WAS EXTREMELY EXCESSIVE AT 56-80 MI OFF. SEEMS THE #3 IRS WENT S. MIA AVIONIC TECHNICIANS GOT ON THIS SIMULTANEOUSLY AND REPLACED THE COMPLETE GYRO PLATFORM.