For the 10th straight year, the Sooners lost the series opener to the Longhorns, falling 7-6 in 12 innings Friday night.
Texas plated the go-ahead and eventual winning runs in the top of the 12th on a two-out, two-run double down the left field line by left fielder Jonathan Walsh.
Catcher Tanner Toal countered with a homer in the bottom of the 12th, but OU could not get another base runner to reach and fell just one run short.
Pitcher Jordan John, who threw 107 pitches, got the tough luck loss after allowing a pair of earned runs and eight hits, while striking out six and walking two in six innings.
"Yeah, he pitched his tail off," said head coach Sunny Golloway. "But the good thing about that is he can be a starter for us and basically he had to pitch like a starter tonight."
Texas' Hoby Milner got the win, while Parker French got his second save of the season.
Second baseman Jack Mayfield gave the Sooners an early 2-0 lead in the bottom of the first inning when he slammed the first pitch he saw over the left field wall.
But for the third straight game, OU blew a lead.
"Same thing, exact same thing, I'm not for sure [what the deal is], you know," Golloway said.
As it was, Texas left fielder Jonathan Walsh reached on the play and right fielder Mark Payton scored, trimming the deficit to 4-1.
But Sooner starter Dillon Overton stranded the bases loaded without any more damage.
Texas third baseman Erich Weiss blasted a solo shot over the right field wall in the top of the fifth, which cut it to 4-2.
He recorded his second RBI of the day on a double in the seventh, and it was 4-3.
Walsh then singled and brought home Weiss, tying things up.
Designated hitter Kevin Lusson gave the Longhorns their first lead later in the inning with a sacrifice fly to left.
Right fielder Cody Reine hammered a game-tying jack in the bottom of the ninth to send it to extras.
"I just saw fastball," Reine said. "He's [Milner] been throwing fastballs. He throws hard, and he's going to try to beat you with a fastball. I just worked on getting my foot down, timed it and it worked."
Each team had a chance there with the bases loaded, Texas in the 10th and OU in the 11th.
OU's golden opportunity came with one out in the bottom of the 11th, but Matt Oberste grounded into a 5-2-3 double play, spoiling it.
"I'd like for him to offer at that first pitch fastball that he saw and he took the gut pitch there and he swings at a ball out of the zone," Golloway said. "It's a two-strike pitch, kind of jams him inside and they're able to roll the double play, so we learn from those things."
The Sooners fall to 0-1 in Big 12 play and 11-7 overall, while Texas is now 1-0 and 8-8 in Big 12 and overall play, respectively.
They'll meet again at 2 p.m. Saturday.