McGruder was 10 of 13 from the field and hit all five of his 3-pointers after the break, including two in the closing minutes. The first answered seven straight points by the Cowboys, and the second gave the Wildcats (12-2, 1-0 Big 12) a 70-65 lead with 1:37 left.
Le'Bryan Nash missed at the other end, and despite two rare misses from the foul line by Will Spradling, the Wildcats had enough to hang on in coach Bruce Weber's Big 12 debut.
Nino Williams added a career-best 17 points for Kansas State, which welcomed back guards Angel Rodriguez and Martavious Irving after missing two games with injuries. Both provided quality minutes down the stretch as the Wildcats beat the Cowboys for the third straight time.
It wasn't because they managed to shut down Smart, Oklahoma State's star freshman.
The athletic guard scored on an assortment of driving layups, pull-up jumpers and free throws for a season-best 25 points. He was coming off a 23-point performance on Monday night, when the Cowboys lost 69-67 to No. 10 Gonzaga in Stillwater, Okla.
Markel Brown added 19 points for the Cowboys (10-3, 0-1), but he and Smart didn't get much help. The rest of the team managed just 23 points as Oklahoma State was dominated on the boards.
It was a typical rough-and-tumble Big 12 game from the opening tipoff.
How much so? The Cowboys got called for a foul after 2 seconds.
There was plenty of pushing and shoving, and more than once it looked as if punches were going to be thrown between teams hoping to contend with No. 6 Kansas for conference supremacy.
Smart's ability to get into the lane—which included a highlight-reel, 360-degree slam in transition—and Brown's scoring prowess allowed Oklahoma State to squeeze out to an early lead.
Kansas State answered with a 12-2 run, forcing Cowboys coach Travis Ford to call a timeout with just under 8 minutes left in the first half. That coincided with a third foul on forward Philip Jurick, taking away one of Oklahoma State's only post presences.
Nevertheless, the Cowboys answered with eight straight points, and the teams traded punches—figuratively, at least—the rest of the half. Oklahoma State wound up leading 32-30 behind 23 points from Brown and Smart, its only reliable offensive production.
McGruder took over to start the second half by scoring the Wildcats' first nine points and helping them slowly pull out to a lead. But then the game truly got into a back-and-forth affair, with two of the league's best defensive teams playing very little defense for a 10-minute span.
Williams and Smart managed to get into a game of personal one-on-one, too. Both of them had a stretch of nine consecutive points for their teams, playing each other to a standstill.
The game remained tight until the final stretch, when McGruder heated back up and hit a 3-pointer with the shot clock winding down for a 62-56 lead with 6:57 remaining.
Two more 3-pointers by McGruder in the final minutes put the game away.