O'Brien closed in on his world-record pace after the first four events of the decathlon in hot and humid conditions at Petrovsky Stadium.
After three lackluster events that had left him 145 points behind schedule, O'Brien surged back on track with a career-best effort of 2.20 meters (7 feet, 2 1/2 inches) in the high jump.
That gave O'Brien 3,814 points after four events, only 21 fewer than he accumulated at the same stage en route to his world record total of 8,891 in 1992.
Earlier, O'Brien won the 100-meter dash in 10.49 seconds, the long jump at 7.81 meters and the shot put at 15.70 meters.
O'Brien was hoping to make a run at becoming the first decathlete to break the 9,000-point barrier.
"If the conditions are good here, I'll have a shot at the world record," O'Brien said beforehand. "I'll be disappointed if I don't score over 8,800 points."
Other key events on Thursday's program were the men's 200 meters, the men's 110-meter hurdles and the women's long jump.
The 200 meters featured 1993 world champion Frank Fredericks of Namibia, silver medalist John Regis of Britain and last year's 400-meter champion Michael Johnson.
World record-holders Colin Jackson of Britain and Heike Drechsler of Germany headed the lineups in the sprint hurdles and women's long jump, respectively.
Decathlon events will be concluded Friday with the 110 meters hurdles, discus, pole vault, javelin and 1,500 meters competitions.
In triathlon, Simon Lessing of Britain won the men's title with a time of 1 hour, 55 minutes, 32 seconds for the 1,500-meter swim, 40 kilometers of cycling and 10 kilometer running. Isabelle Mouthon of France took the women's event in 2:09:34.8.
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