That finished off Woods, and the second-longest streak in tour history.
"I was so enthralled with the line, I didn't see the ball mark," he said. "I knew if I hit it left-center, the match would be over. It's my fault for not paying attention to detail."
It is rare that Woods makes such a gaffe, and misses such a short putt with so much riding on the outcome. One hole earlier, he hit a sand wedge to 5 feet and made the birdie putt to extend the match, completing a rally in which he was four holes behind after seven holes, and still trailed by three with eight holes left.
O'Hern became the first player to beat Woods twice in professional match play. The short, straight-hitting lefty also beat Woods in the second round at La Costa two years ago.
Byron Nelson's record of 11 straight tour victories in 1945 again looks as untouchable as ever.
Woods had won seven in a row _ although the purity of the streak was in question because he had failed to win four times during that streak while competing on other tours _ and he will have to start over next month in the Arnold Palmer Invitational at Bay Hill.
"Tiger being Tiger, he birdied the 18th," O'Hern said. "I thought it was 'Game Over,' then he gave me a break."
O'Hern advanced to play Henrik Stenson, who defeated Aaron Baddeley, 4 and 3, in the shortest match of the third round. Stenson (No. 9) is the top seed still alive in this fickle tournament.
The lone American remaining is Chad Campbell, who advanced to the quarterfinals for the second straight year after beating David Toms. He will play Stephen Ames, a 3-and-1 winner over Stewart Cink.
In other matches:
_Paul Casey beat Shaun Micheel for the second time in five months, 2-up. Casey also beat the former PGA champion in the final of the HSBC World Match Play Championship in September, where Woods' worldwide winning streak ended at five.
_Defending champion Geoff Ogilvy won his ninth straight match, defeating Niclas Fasth of Sweden, 2 and 1.
_Justin Rose built a big lead and beat good friend Charles Howell III, 3 and 2.
_Trevor Immelman never trailed in an otherwise close match against Ian Poulter, winning 2 and 1.
For most of a cool, blustery day in the high desert at The Gallery, it seemed as though only one match was in progress, and a huge crowd that packed every grandstand filled one side of every fairway could not believe what they were seeing.
Woods hardly looked like a player who had won every PGA Tour event he entered since he was runner-up at the Western Open in July.
He missed a 4-foot putt on the third hole that would have given him an early lead, and then it fell apart in a hurry.
Woods' tee shot flared into the wind and dropped into the water on his way to double bogey on the fourth hole. He hit another drive on No. 6 that landed at the base of a saguaro cactus, and he took two shots to move the ball 35 feet out of the desert in taking another double bogey. On the seventh, Woods' drive landed at the base of a desert shrub, and he blasted that over the green to fall four holes down.
"It was a struggle," Woods said. "I just didn't have control of my golf swing. I had a two-way miss going today."
But he pecked away at the lead, building momentum along the way.
He won his first hole with a 15-foot birdie at No. 8, then rolled in another 15-foot birdie on the 11th and showed his first emotion with a 7-foot birdie on the 12th that brought him to within one. He finally squared th match on the 15th when O'Hern's approach landed on a shelf and tumbled down into a swale, from where it took two chips to reach the green.
But he lost a golden chance to seize control on the 601-yard 17th, which played downwind. Woods hit a 3-iron on the toe of the club, and it landed some 70 yards short in a bunker. He had to settle for par, while O'Hern knocked in an 8-foot birdie to take a 1-up lead to the 18th. Woods didn't go easily, hitting a 342-yard drive that left him a sand wedge to the green.
The only player more stunned than Woods over his short miss on the 19th hole was O'Hern.
"My caddie gave me another ball and said, 'OK, next hole,'" O'Hern said. "I said, 'Mate, he doesn't miss these.'"
He missed this one.
Woods pulled a 4-iron well left of the green, and his chip came up 15 feet short. He missed on the low side, and O'Hern's par putt just curled in the right side.
"To beat him once was an amazing thrill," O'Hern said. "I'm sure he wanted to even the score today. I just knew if I played well and played solidly, I could do it again."
The size of Woods' advantage off the tee can best be summed up in club selection. O'Hern hit 26 metal clubs over his 20 holes _ driver, fairway metal or hybrid _ to Woods' 18.
Yet it doesn't matter when the Aussie stripes it down the middle. Not only has he beaten Woods twice, he has gone 37 holes with the world's No. 1 player in match play and has yet to trail.