WDB ACFT ON OCEANIC ROUTING WAS OFF COURSE DUE TO WAYPOINT ERROR ENTERED IN THE FMC. RPTR CAPT ELECTED TO GO DIRECT NEXT WAYPOINT; BUT 500 FT OFF ALT. APPARENTLY THEY WERE NOT IN COM WITH OCEANIC.

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IN CRUISE ON THE OCEANIC PORTION OF THE FLT; I RETURNED FROM MY REST BREAK AND WAS ADVISED THAT THE RELIEF FO PLOTTED A FIX AND CONFIRMED THAT WE WERE OFF COURSE. I RECHKED HIS WORK AND THE PLOT MADE BY THE OPERATING FO. WE WERE OFF COURSE. BY FLYING TO THE WAYPOINT; WHICH WAS PRESENTLY ACTIVE; WE WOULD BE ON COURSE AND ARRIVE AT THE NEXT WAYPOINT; WHICH WAS AS CLRED AT THE TIME GIVEN IN THE LAST POS RPT. I BELIEVED THE SAFEST COURSE OF ACTION TO BE CONTINUING TO THE WAYPOINT AT AN ALT 500 FT LOWER THAN THE CLRNC ALT AND TO CONTINUE TO MAKE FULL USE OF THE TCASII. AT ALL TIMES WE WERE MONITORING 121.5 AND 131.8 PER OCEANIC PROCS. NO ACFT WERE SIGHTED AND NONE WERE ON THE 2 FREQS. THE WAYPOINT ERROR MUST HAVE BEEN INTRODUCED DURING THE WAYPOINT LOADING. THE ERROR WAS NOT DETECTED BY EITHER MYSELF OR THE OPERATING FO. I LATER FOUND THAT HIS PLOT OF POS; WHICH WOULD HAVE SHOWN THE ERROR EARLIER; WAS IN ERROR SHOWING US TO BE ON COURSE. MY ONLY EXPLANATION FOR THE ERROR GOING UNDETECTED IS OUR EXPECTING TO SEE THE 'NORMAL' NUMBERS. I CHKED THE WAYPOINTS AT LEAST TWICE AND OBSERVED THE FO DOING THE SAME. I HAVE NO OTHER EXPLANATION. THE SHOCK OF THIS TYPE OF ERROR IS DEVASTATING.