Andrea Riley, Oklahoma State's streaky 5-foot-5 senior, made six of her first seven shots but missed her last nine and wound up with 32 points - giving her a tournament-record 112 in just three games.
The victory gave the Sooners (23-9) a three-game season sweep of the Cowgirls (23-10) and pits them against an Aggies team that handed Nebraska its first loss of the season in the other semifinal. A&M and Oklahoma split two games in the regular season.
Robinson had 19 points for the Sooners, Thompson 17 and Nyeshia Stevenson 15.
Robinson's two free throws gave Oklahoma a 70-64 lead with 40 seconds to go in the back-and-forth affair between state rivals, then Tegan Cunningham made a 3-pointer and Oklahoma State trailed by three.
Thompson, fouled immediately, sank both her foul shots. After Cunningham grabbed Riley's missed 3-point attempt and put it in, Robinson calmly sank two more foul shots with 7 seconds left.
The Cowgirls started the game on a 16-4 run but the Sooners caught up right after the start of the second half when Stevenson hit eight quick points, including two 3-pointers for a 46-43 Oklahoma lead.
In the first half, Riley canned two long jumpers and Cunningham sank a pull-up jumper as Oklahoma State seized a 36-25 lead. Robinson, after throwing up an air ball, hit a bucket and made two foul shots to help cut the halftime lead to four points.
Riley, whose 43 points against Iowa State on Friday set the single-game tourney record, was 12 for 30 from the floor and only two of nine beyond the arc. Cunningham had 21 points for the Cowgirls.
Abi Olajuwon's three-point play put the Sooners on top 64-62 with 6:09 to play and the Cowgirls never caught up. A bucket by Carolyn Blair-Mobley cut it to 66-64 but Robinson sank an uncontested bucket a moment later.
The Sooners were 22 of 26 from the line.