Carolyn Davis made a contested layup with 12:22 left that capped an 8-0 spurt and gave the Jayhawks a 49-47 lead, their first since early in the first half. Oklahoma State (22-9) responded immediately by scoring the next seven points and never trailed again.
Riley set the school record by playing in her 129th game, and also became the Cowgirls' career assists leader. She can add to her total against Iowa State in the quarterfinals on Friday.
Davis had 31 points for Kansas (15-15), which lost its final six games.