The decathlon may be the Olympics’ defining event. On the field, it is unglam-orous – 10 grueling events performed over two days before sparse crowds and scored in incomprehensible fashion. But in victory, it is heroic. The gold-medal winner carries the mantle of ““world’s greatest athlete’’ and, in America, joins a roster of sporting immortals from Jim Thorpe to Bob Mathias to Rafer Johnson to Bruce Jenner. O’Brien, who will turn 30 the day before the Atlanta Games open, knows that his world record and his three world titles will never be validated without the Olympic crown. ““All I ever wanted people to think was that I was the world’s greatest decathlete,’’ he says. ““But people aren’t paying attention until the Olympics.''

On Saturday afternoon, he was ready. He had learned the difference between being hot and being warmed up. He had worked with a sports psychologist, who trained him to ““free up his mind’’ to focus on one vault at a time. He had switched to a softer, ““more forgiving’’ pole and would start vaulting at 14 feet 9 inches, a foot lower than last time. Still, the painful memory hovered, and Dan admitted, ““Every time I step on the pole-vault runway I feel that anxiety.''

His coach, Mike Kelley, was feeling it, too. He watched from the stands, worried that if Dan missed – ““and his first vault, no matter what height, is always ugly’’ – the pressure would get even more intense. Finally O’Brien rocked on his heels once, lifted the pole and charged down the runway. Ugly or elegant, it didn’t matter when Dan soared over the bar. As he bounced up off the mat, O’Brien pumped his fists in exuberant triumph. The modest jump meant certain victory. Dan would go on to win the decathlon by 90 points and, most important, a return trip to Atlanta next month. ““I can go home and work on a few things and come back here in July and give the world record a try.''

O’Brien’s victory was the most dramatic moment in a week of high drama. America boasts the deepest track-and-field talent in the world, and a few centimeters or hundredths of a second are often the difference between going to the Games and going home for the summer. Long-jump world-record holder Mike Powell barely escaped disaster. On his final attempt, he leaped from sixth place to first – and onto his third Olympic team. ““This meet is tougher than the Games,’’ he said. ““Making the team is so stressful. I’ve faked sleeping for the past three days.''

During the trials, some of the world’s greatest athletes began to show their age. Both Carl Lewis, who will turn 35 next week, and Jackie Joyner-Kersee, 34, claimed to be in the best shape of their lives, but both struggled to make the team. Joyner-Kersee, who has won the last two Olympic gold medals in the heptathlon, had an apparently insurmountable 116-point lead going into the last of the seven events, the 800-meter run. But she staggered to the finish line way behind the rest of the pack, seemingly on the verge of collapse, and fell to second place behind 25-year-old Kelly Blair. Though she earned an Olympic spot, Joyner-Kersee was dismayed by her first heptathlon loss since the 1984 Olympics. ““I’m not going out this way,’’ she pledged. ““I’ve worked too hard this year and I’d like to see it pay off.''

Lewis, too, has been working incredibly hard. But he hadn’t run four 100-meter races in 90-degree heat and 90-plus percent humidity. Though he reached the finals, he cramped up on his first step out of the blocks and finished last. Still, Lewis refuses to believe that age had anything to do with his disappointing performance. ““I’m frustrated because I know I can still run fast,’’ said the winner of eight Olympic gold medals. His best hope remained the event ““that has never deserted me’’ – the long jump. ““I have to go out and jump like crazy,’’ he said.

Instead, his jumping probably made him a bit crazy. In winning the last three Olympic long jumps, Lewis had always surpassed 28 feet at the trials. This time, however, his best leap was 27 feet 2 and 3/4 inches, barely good enough for third place. But he’s going to the Games, the first man ever to make five consecutive Olympic teams. Lewis, an athlete noted for his brash style, was subdued and genuinely appreciative of how ““at a time when it was easy to turn away, everybody rallied to my support.''

Now comes the hard part. Atlanta’s climate does not improve in July. Nor will the facilities. The practice track is a half mile from the Olympic Stadium. Competitors were forced to warm up, then ride an air-conditioned bus to the match. Cramping became the rule, not the exception. ““At 50 meters every muscle in my legs started cramping,’’ said Dennis Mitchell, who won the 100 meters in a blistering 9.92. ““I said, “If I move fast enough, they won’t catch’.’’ They didn’t, until after the finish line, when Mitchell collapsed.

America’s two premier track stars of the past year, Michael Johnson and Gwen Torrence, breezed through their first events. Johnson ran the third fastest 400-meter race in history, while Torrence thrilled her hometown crowd with her fastest 100 meters ever – 10.82 – to beat Barcelona gold-medalist Gail Devers.

By Saturday night, O’Brien had little energy to muster anything but relief. ““What happened in ‘92 was so devastating, you can’t even comprehend it,’’ he told NEWSWEEK. ““There’s a feeling of disbelief – almost like you’re in a dream. But I came to understand that’s just athletics. Kim Zmeskal falls off the balance beam. Mike Tyson gets knocked out. So many people fail and quit. Well, it’s OK to fail as long as you don’t give up.’’ He waved to the crowd and walked out of the stadium listening to the second loudest noise fans can make: cheers.