North Dakota State guard Carlin Dupree entered the game for the first time midway through OT and calmly delivered two clutch free throws -- never mind his 58.3 free-throw percentage this season -- to put the Bison up 74-72. After an Oklahoma turnover, Dupree gave the Bison a four-point lead with a double-clutch layup over two defenders that skimmed over the side of the rim.
Bison guard Lawrence Alexander led North Dakota State with a career-high 28 points. North Dakota State made 20 of 22 free throws.
Twelfth-seeded North Dakota State (26-6) advances to face either fourth-seeded San Diego State or 13th-seeded New Mexico State on Saturday.
Every time it appeared Oklahoma was on the ropes, senior forward Cameron Clark pulled the Sooners back up, tying the game at 70 with a 3-pointer two minutes into overtime and knotting the game again with 1:18 remaining in OT with two free throws following Bison guard Taylor Braun's fifth foul. Clark paced Oklahoma with 25 points on 9-of-21 shooting.
Alexander nailed a 3-pointer in front of his North Dakota State bench to tie the score at 66 with 11.2 seconds left in regulation. Oklahoma missed three shots in the final 9.8 seconds, sending the game to overtime.
Sooners guard Isaiah Cousins (17 points) made two free throws to give Oklahoma its first lead of the second half with 1:30 to play.
After Braun's lefty layup rolled out at the other end, Clark splashed a 17-foot jumper for his seventh point in 2:30 as part of an 8-0 run staked the Sooners to a 65-61 advantage that appeared safe.
However, an ill-advised foul on Cousins sent Braun to the line, and he made it a two-point game with 32.4 seconds left. Cousins made only one of two free throws on the Sooners' ensuing possession to keep the door open for North Dakota State. Braun drew three defenders in the middle of the lane and found Alexander for the 3-pointer.
The Bison shot 64 percent in the first half but went frosty in the second half.
Alexander carried the Bison in the first half with three of North Dakota State's five 3-point field goals in the first 20 minutes of the game.
Braun, the Summit League Player of the Year, was held in check in the first half with just five points.
Oklahoma shot under 38 percent in the first half and trailed 37-29 at the break.
NOTES: G Taylor Braun, one of three senior starters for North Dakota State, led the team in scoring (18.6 points per game) and rebounding (5.5 per game) this season. ... North Dakota State led the NCAA in shooting (50.1 percent). ... Oklahoma coach Lon Kruger was voted Big 12 coach of the year. He is the only coach in history to lead five different schools to the NCAA Tournament.