The 28-year-old American was a tired figure as he jogged in after the 1500m, an event he hates. Earlier, he had looked on course to provide the athletics programme with a spectacular finish.
O'Brien ended the first day 16 points up on a record schedule and had stretched that lead to 29 after the hurdles and discus. But his troubles started in the pole vault: he cleared 4.90 metres comfortably, electing to pass on 5.00m, but then failed at 5.10m.
That left him adrift by one point and, despite throwing 62.20m in the javelin after two indifferent efforts, he went into the final event seven points down on the record. The double world champion's target was 4:40.92, but O'Brien was always behind the clock and he finished 30 seconds outside the target.
So he had to settle for a Games record total of 8,715 - 176 outside the world record he set in Talence, France, in 1992. O'Brien said: 'It was disappointing to leave two days' work behind with a 1500 like that. But it is an event not many decathletes enjoy and I don't yet have the mental toughness it needs.'
The anticlimax left the crowd having to settle for a mighty leap of 2.40m by Javier Sotomayor in the high jump and a Carl Lewis-anchored sprint relay win by a Santa Monica Track Club quartet for the United States.
The world championship bronze medallist, Jonathan Edwards, was never in contention in the triple jump. He was last of seven with 16.57m, so Britain finished the Games with two golds from the hurdlers, Colin Jackson and Sally Gunnell; silvers from Tony Jarrett - second to Jackson - and, in the 400m, Du'aine Ladejo; plus bronzes for John Regis in the 200m and Jon Brown in the 1500m.Reuse content