OSU gave up 22 offensive rebounds to the Bears, who won the battle on the boards 51-36.
"It's difficult to win a game when you've only got two people scoring, but we had no other options," Cowboys head coach Travis Ford said of Markel Brown (26 points) and Phil Forte (20). "For a stretch there they pretty much locked those two guys up and we couldn't get anything else, and you can't win giving up 22 offensive rebounds. We were bad.
"We had guys fighting, they played hard. I was proud of their effort. We just didn't get enough contributions."
Despite an outstanding performance from Brown (26 points, 6 assists, 4 rebounds and no turnovers while playing all 45 minutes), the Cowboys let a lead slip away in the game's final five minutes. In six of their nine Big 12 losses this season the Cowboys have lost after leading in the last five minutes.
"I thought Markel played like a big-time player and Phil Forte gave it everything he had," Ford said. "They fought hard. It just wasn't enough."
The Cowboys were playing again without Marcus Smart, who was serving the final game of a three-game suspension for going into the stands to push a fan at Texas Tech on Feb. 8.
OSU led by as many as 10 points after Forte's three-pointer just a minute into the second half but the Bears kept chipping away and eventually pulled even at 50-50 with a little under six minutes to play in regulation.
Brown's layup and another Forte three-pointer put the Cowboys on top 55-54 with 3:20 left. But the Forte trey would be their last basket until an improbable buzzer beater by freshman Leyton Hammonds to send the game into overtime.
Trailing 58-55, it looked hopeless for the Cowboys when Forte's desperation three-pointer bounced off the rim and out of bounds to Baylor with 3.5 seconds remaining in the game. But Baylor's Gary Franklin made a bad pass and Hammonds intercepted it before dribbling to the wing and launching a three-pointer right before the buzzer sounded. It went in, silencing the Baylor crowd, to send the game into overtime. It was Hammonds' first made three-pointer since the season opener against Mississippi Valley State on Nov. 8.
Baylor's Cory Jefferson hit a three-pointer on the first possession of overtime to give the Bears the lead and they never looked back. Jefferson, who had 25 points and 13 rebounds, has become a thorn in the side of the Cowboys the past two seasons, recording a double-double in all five games.
The Cowboys pulled within 66-64 in overtime on Brown's three-pointer from the top of the key with 1:18 left but Kenny Chery's driving layup as the shot clock was winding down put the Bears back up by four. Jefferson's two free throws with 20 seconds remaining sewed it up as the Bears swept the regular season series.
"We didn't defend in overtime," Ford said.
Brown and Forte were outstanding as they combined for 46 points, 8 assists, 8 rebounds and no turnovers while playing 45 and 44 minutes, respectively.
The Cowboys will look to end the seven-game losing skid against Texas Tech on Saturday afternoon at Gallagher-Iba Arena.