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Jeff Roberson, Associated Press
Ohio forward Ivo Baltic (23) looks down during the second half of an NCAA tournament Midwest Regional college basketball game against North Carolina Tar, Friday, March 23, 2012, in St. Louis. North Carolina won 73-65 in overtime.

ST. LOUIS — Ohio came within a free throw of pulling off one of the biggest upsets in NCAA tournament history.

Trying to become the first team seeded 13th or worse to make the regional finals since the tournament expanded in 1985, the Bobcats could have taken the lead over top-seeded North Carolina on a three-point play with 25 seconds left in regulation. But Walter Offutt missed from the line, and Ohio went 0 for 6 from the field as the Tar Heels escaped with a 73-65 victory in the first overtime game of this year's tournament.

"One free throw away," Offutt said. "As a leader on this team, I take responsibility that I got to hit that free throw. ... It just feels terrible to kind of let my team down in that sort of way."

The Bobcats (29-8) have nothing to be ashamed of, though.

The last of the so-called "little guys" in the tournament, they gave North Carolina everything it could handle and then some.

And the Tar Heels knew it.

"We feel like we got away with one," said Tyler Zeller, who led North Carolina with 20 points and a career-high 22 rebounds, one of three Tar Heels to finish with a double-double. "Ohio played the better game, they hit a lot of shots. I think we just were able to make a lot of plays at the end that made us capable of pulling it out."

Offutt led the Bobcats (29-8) with 26 points, including 18 from 3-point range, and Nick Kellogg added 14. But D.J. Cooper, who had averaged 20 points in the first two tournament games, finished with just 10 on 3-of-20 shooting.

Reggie Bullock had 17 points and 10 boards, and John Henson had 14 points and 10 rebounds. Harrison Barnes finished with 12, five below his season average, but he scored five in overtime.

"Probably the ugliest win I've ever been a part of," Bullock said.

Though North Carolina was playing without star point guard Kendall Marshall, the Tar Heels made the most of their decided size advantage early. Zeller and John Henson pounded the ball inside possession after possession as the Tar Heels (32-5) jumped out to a double-digit lead midway through the first half — and it looked as if this would be one more in a series of lopsided games between the haves and have-nots.

But the 13th-seeded Bobcats got hot from long range, going 8 of 13 from 3-point range in the second half. Offutt did most of the damage, making three 3-pointers during a 14-5 run that pulled the Bobcats to 42-41 with 10:23 to play.

"We didn't really change anything at halftime," Kellogg said. "Coach just told us to continue to be shot ready because we knew we were getting pretty decent looks there in the first half. They just weren't falling for us. So coach told us to stick with it, continue to be ready to shoot the ball, and eventually they would start falling."

After a Zeller layup and a dunk by James Michael McAdoo, Cooper hit his only 3 of the game. Kellogg came right back with another to give Ohio its first lead, 47-46 with 8:28 to play, and the Ohio faithful — plus all the new fans they picked up — went into a frenzy.

The teams traded leads, neither able to put the other away. When Barnes missed badly on a 3 with 1:17 to play, the crowd taunted him with chants of "Air-ball!" ''Air-ball!" But Williams could be heard yelling "One stop!" at his team, and the Tar Heels got it when Cooper missed on a jumper at the other end.

Bullock then drilled a 3 to give North Carolina a 63-61 lead with 39 seconds left, but Offutt — who else? — scored on an off-balance jumper and drew a foul by Stilman White, Marshall's fill-in, in the process. Offutt's attempt was way left, the only free throw the Bobcats missed all night, and Zeller grabbed the rebound.

"It's just like him to take responsibility for the loss because that's who he is," Bobcats coach John Groce said of his senior, a transfer from Ohio State by way of Wright State. "But the reality of it is, that doesn't fall on him. There's so many plays during the game that make up the game and in a 45-minute game, and if anybody deserved to make that free throw, it's him. It just didn't go in."

The Tar Heels worked the shot clock down to the very last seconds, but Barnes missed yet another shot and Offutt came up with the rebound. He flipped the ball to Cooper, who raced to midcourt and launched a prayer. It caromed off the rim, sending the game into overtime.

Kellogg's father, Clark, calling the games in Atlanta for CBS, smiled as he watched the final plays.

"Overtime," he said, clapping.

But the Bobcats had nothing left. Bullock opened the extra period with a 3 and Barnes came right back with a jumper. After Kellogg made a pair of free throws to cut it to 68-65, Barnes made a pair of his own and Ohio could never get any closer.

North Carolina (32-5), which has won 11 straight in the regional semifinals, plays the winner of North Carolina State-Kansas on Sunday afternoon.

"Awful proud of our guys for the way that they fought," Groce said. "But as I told them in the locker room, there's nothing I can say to them at this point that's going to take away the sting of getting beat in that one. They got a lot of pride and they believed all year that we could compete and beat anybody that we played. That was no different heading into tonight's game."