B767 FO ATTEMPTING TO STOW OXYGEN MASK; BREAKS RED TAB CAUSING MASK TO BE UNSERVICEABLE. FLT CREW DESCENDS TO 10000 FEET AND DIVERTS TO SUITABLE ARPT.

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AFTER THE CAPT'S RESTROOM BREAK; THE FO PROCEEDED TO ATTEMPT TO STORE HIS OXYGEN MASK. THE MOVABLE RED TAB THAT YOU SQUEEZE BROKE AND OXYGEN WAS UNCTLABLY FLOWING OUT OF THE HOLE AT THE BROKEN TAB. THE MASK WAS OBVIOUSLY UNUSABLE. THE FO SECURED THE TAB WITH A RUBBER BAND TO STOP THE DEPLETION OF THE COCKPIT CREW OXYGEN. WE IMMEDIATELY REQUESTED A DSCNT TO FL250; THEN TO 10000 FT. CONSIDERING OUR POS WAS OVER GCK; WE FELT THAT DEN WOULD BE A SUITABLE DIVERSION DEST. WE INFORMED DISPATCH AND THEY REPLIED WITH THE SUGGESTION BY DISPATCH TO PERHAPS HAVE THE FO USE THE FIRST OBSERVERS OXYGEN MASK. WE DID NOT FEEL THAT WOULD COMPLY WITH THE FLT MANUAL; AS IF WE HAD AN EMER THE QRC CLRLY STATES 'CREW COMS ESTABLISH.' THIS WOULD BE IMPOSSIBLE TO HAVE THE FO IN HIS SEAT. HE WOULDN'T BE ABLE TO COMMUNICATE. ALSO; WE WERE CONCERNED THAT ALTHOUGH THE FO STOPPED THE CREW OXYGEN FROM BECOMING DEPLETED BY APPLYING A RUBBER BAND TO THE MASK; THAT IF THE RUBBER BAND BROKE (LIKE THE FIRST ONE DID!); NONE OF US IN THE COCKPIT WOULD HAVE OXYGEN. WE DIVERTED UNEVENTFULLY TO DENVER.