The victory advances the Cowgirls (16-14) to a quarterfinals game against second-seeded Baylor on Friday.
Hardeman, who finished with 12 points, hit four consecutive shots from 3-point range early in the second half that helped Oklahoma State take a 33-32 lead after trailing 28-17 at halftime and 15-0 to start the contest.
Andrea Riley added 16 points and eight assists for the Cowgirls.
Texas Tech (16-15) got 19 points from Maria Moore and 11 each from Jordan Murphree and Kierra Mallard.
In losing its fourth game in five starts, Tech hit 9-of-36 shots in the second half.