Carlin Dupree came off the bench when Taylor Braun, the Summit League Player of the Year, fouled out with 1:18 remaining in the extra session.
Dupree hit a pair of free throws seconds after stepping onto the floor, which gave North Dakota State a lead, then followed that with a tough layup in traffic to help the No. 12 Bison to an 80-75 upset win over fifth-seeded Oklahoma in the second round of the NCAA Tournament.
Lawrence Alexander finished with a career-high 28 points to pace North Dakota State (26-6), while Marshall Bjorkland scored 14 points with six rebounds and Braun tallied 11 points on just 3-of-11 shooting.
Cameron Clarke scored 22 of his 25 points in the second half and overtime in his final game for Oklahoma (23-10) and Isaiah Cousins netted 17 points with eight rebounds.
Jordan Woodard added 13 points and Buddy Hield, a second-team All-Big 12 selection, contributed nine points on 4-of-14 shooting.