WDB ACFT ON OCEANIC FLT ENTERS WRONG INFO INTO OMEGAS RESULTING IN A GROSS NAV ERROR.

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INBOUND ACFT WAS 1 1/2 HRS LATE ARR SNN ARPT; NORMAL PREFLT DUTIES WERE ACCOMPLISHED. FO INITIALIZED OMEGAS; I CHKED AND FOUND CORRECT. WHILE FO WAS DOING HIS PAPERWORK I ENTERED COMPUTER FLT PLAN OCEANIC WAYPOINTS. WE BOTH THEN CHKED AND XCHKED WAYPOINTS; COURSE AND DISTANCE. I CALLED SHANWICK CTL FOR CLRNC. SOMETIME LATER SHANWICK CAME BACK WITH OUR CLRNC; WE ALL LISTENED AND WROTE DOWN CLRNC. IT WAS SLIGHTLY DIFFERENT FROM OUR FILED ROUTING. I TOLD THE FO TO CORRECT OUR FLT PLAN AND I PROCEEDED TO INSERT NEW WAYPOINTS IN OMEGAS. DURING THIS TIME I ALLOWED MYSELF TO BECOME DISTR DUE TO SOME MAINT DEFERRALS AND NEVER GOT BACK TO CORRECTING OMEGAS. WE TOOK OFF AND PROCEEDED ON COURSE NORMALLY. WE DID ALL THE CHKS AS PER OUR NAV MANUAL. SINCE THE FIRST 2 WAYPOINTS WERE THE SAME AS ORIGINAL FLT PLAN EVERYTHING PLOTTED CORRECTLY; UNTIL AFTER OUR 20W RPT. SINCE I WAS FLYING THIS LEG; THE FO GAVE THE POS RPT AT 20W AND I THOUGHT I HEARD HIM SAY '54N/40W NEXT.' AFTER HIS 54N/30W ESTIMATE I ALSO THOUGHT I HEARD SHANWICK READ BACK THE SAME WITH A COPY TO GANDER. APPARENTLY NOT SO. THE 10 MINS PAST PLOT AFTER 20W WAS RIGHT ON COURSE BECAUSE I PROBABLY READ WRONG N COORDINATES TO FO. AT THE 54N/30W POS RPT AGAIN I HEARD WHAT I WANTED TO HEAR BUT SOMETHING JUST DIDN'T SEEM RIGHT. I THEN RECHKED EVERYTHING AND FOUND WE WERE AT 54N/30W INSTEAD OF 53N/30W. AT THIS TIME I STARTED CORRECTING TOWARD 53N AND CALLED GANDER FOR RTE CLARIFICATION AND TO ADVISE THEM AS TO WHAT I HAD DONE. WE WERE THEN CLRED DIRECT TO 53N/40W. LATER WE WERE SELCAL'ED AND ADVISED THAT A RPT WOULD BE FILED. ON GND GANDER; I TELEPHONED GANDER OCEANIC TO EXPRESS MY CONCERN AND TO INQUIRE AS TO ANY TFC CONFLICT. THERE WAS NO OTHER TFC IN AREA. I HAD JUST GOTTEN OVER A FAIRLY BAD HEAD COLD WHICH I BELIEVE MAY HAVE CONTRIBUTED TO MY INATTN TO DETAIL.