Clark (71), along with third-round leader Tim Herron (72) and Charlie Wi (68) finished 10 under. Steve Flesch (68), Bob Heintz (68), Jeff Maggert (69) and Kenny Perry (71) were six strokes back.
Clark squandered a two-stroke lead on the back nine and he and Herron, playing in the final group, heard the gallery's roar after Ogilvie's eagle.
Ogilvie, at 11 under, had to carry his wedge shot over water 119 yards to the pin. The ball hit the middle of the green and spun back about 30 feet to the cup, where it dropped.
Ogilvie had a wide-eyed look, jumping up and yelling 'Whoa!' before an awkward high-five with his caddie to celebrate the lucky shot that moved him 13 under.
He added a birdie on No. 18 to finish his round, but the eagle was all Ogilvie needed.
The 33-year-old Duke graduate who closely monitors the stock market had made seven consecutive cuts and led here at the midway point, but said his weekend play made him a seller, not a buyer.
Now he'll have $720,000 to invest for winning at the Brown Deer Golf Course, the PGA's shortest event, after the economics major vastly underestimated his own value on the final four holes, which he played at 9 under for the tournament.
The holes saved him on Saturday. In the midst of a poor round, he recorded an eagle-birdie sequence on Nos. 15 and 16 to move back within a shot of the lead heading into Sunday.
On Sunday, Ogilvie bogeyed the 10th to fall two off the lead, but he responded with a 24-foot birdie on No. 13 to move to 11 under and set the stage for his eagle.
It was more frustration for Clark.
After squandering a three-stroke lead on the back nine at the John Deere Classic last week, the 31-year-old South African who now lives in Scottsdale, Ariz. held a two-stroke lead for a fleeting moment after Ogilvie's mistake on the 10th.
But Clark, runner-up at the 2006 Masters, missed a 16-footer for par on the 10th and dropped to 11 under and bogeyed No. 16 to fall further behind.
After receiving a cortisone shot three weeks ago for a bulging disk in his neck, Clark has been playing some of his best golf lately in an effort to make a third consecutive Presidents Cup team and the FedExCup playoffs.
But he is still winless on the PGA TOUR in part because he failed to make a birdie on the back nine.
Herron, who has more than $14 million in career earnings despite just four PGA TOUR titles, couldn't contain his wayward drives and the shots came back to haunt him after he took a one-stroke lead into the final round.
Herron, 37, carded two bogeys on the front nine and continued to squander opportunities on the back side. Herron finally birdied No. 16, but gave it right back with a bogey on No. 17 leaving no one to challenge Ogilvie.
Earlier in the day, Wi birdied four of the last five holes to move to 10-under, but it wasn't nearly enough to catch Ogilvie, either.
The U.S. Bank Championship had typically been played the week after the British Open, but fell concurrently to the tournament this year and will again in 2008.