The Sooners survived two Kansas State rallies to win 7-6 in 11 innings Saturday at Chickasaw Bricktown Ballpark.
Oklahoma (39-19) swept through pool play undefeated and will face Kansas, TCU or West Virginia Sunday at 1 p.m. in the championship game.
"I think that was just a tremendous baseball game," OU head coach Sunny Golloway said. "That game should speak volumes about Big 12 baseball."
The Sooners and Wildcats entered the game 2-0 in the preliminary round, giving the two control of their pool and path to Sunday's title game. Instead they played as if the championship was on the line Saturday.
Kansas State (46-17) rallied with a four-run ninth to tie the game, then matched Oklahoma's single run in the 10th before the Sooners prevailed 7-6 in 11 innings.
OU junior reliever Kyle Hayes struck out the two batters he faced in the 11th to earn his first save.
"We knew K-State could swing the bats. We knew they were going to score runs, and we felt that they were going to make a run, which they did. When they put up the four runs, there was disappointment in our dugout, but the fact that we threw out the runner at third base (in the ninth inning) was huge to get off the field and back in to score a run."
With runners on first and second with one out in the ninth, Golloway called to the bullpen for Jacob Evans, but the Wildcats feasted on the freshman closer. Tanner Witt hit an RBI single to left field, scoring R.J. Santigate, and Shane Conlon followed with a two-RBI double down the left field line, plating Ross Kivett and Witt to make the score 5-3.
Evans struck out Jared King but then hit Jon Davis with a pitch to load the bases. Mitch Meyer singled into left field, scoring two to tie the game, but Davis was thrown out turning to third by OU centerfielder Max White to end the inning.
White doubled to lead off the 10th inning and Matt Oberste plated the Sooner baserunner in the next at-bat with a single.
"The ups and downs of this game were crazy," White said, "but it was really motivating seeing both teams fight. Two teams just battling like that it is unbelievable. We just have to keep fighting, especially after these games."
Ralph Garza (5-1) hit Blair DeBard to gift a baserunner for the Wildcats in the 10th. After a sacrifice bunt and groundout moved DeBard to third, Kivett floated a base hit over third base to answer the Sooners' run.
Taylor Alspaugh doubled down the left field line, in OU's second at-bat in the 11th, and Craig Aikin hit an opposite-gap single into left to bat in the Sooners' right fielder.
Oklahoma improved to 7-1 in extra-inning games.
Oklahoma loaded the bases with two outs in the fourth inning and Garrett Carey cleared them with a double that extended just beyond right field Davis' reach. Hermelyn singled to right center and then Colt Bickerstaff was walked, sending KSU head coach Brad Hill to the bullpen for Jared Moore. Moore walked Alspaugh before Carey struck next.
OU starting pitcher Jake Fisher pitched back-to-back strikeouts of Shane Conlon and Jared King to escape a bases-loaded situation in the third. The Wildcats loaded the bases again in the fourth before Fisher struck out R.J. Santigate for the first out. Kindle Ladd replaced Fisher on the mound and struck out Lance Miles, but beaned Kivett with a full-count pitch to send home the Wildcats' first run.
OU's defense shutout the opposition for 21 innings, a Big 12 Championship record streak, before Kansas scored in the fourth.
Kansas State loaded the bases in the third, fourth, ninth and 11th innings, but batted 1-for-8 in the situations.
Oberste led off the second inning hit by a full-count pitch and advanced to second on a wild pitch. Oberste reached third on Hector Lorenzana's groundout to second and scored when Hermelyn belted a sacrifice fly to left field.
Kansas State threatened again in the seventh with back-to-back singles to put baserunners on first and second with no outs. But the Sooner defense came through once again. Alspaugh tracked down a long fly ball over his head in right field and nearly put out Kivett, who was running to third after assuming Alspaugh had no chance at the play. The Sooners turned a 6-4-3 double play, just getting out King on a bang-bang play at first.
Oklahoma fell to Missouri 8-7 in last year's Big 12 Championship title matchup.
"This team has been through a lot," Golloway said. "We aren't going to apologize for being in championship positions. We have come up short three times, and tomorrow gives us our first opportunity within one calendar year to not come up short again."