McClellan led the Longhorns with 25 points after coming off the bench for the fourth straight game. Barnes called it "one of the best games he's played just because of the way he was engaged mentally."
It still wasn't enough to keep Texas from losing its first five league games since doing it in back-to-back seasons in the Southwest Conference in 1975 and 1976 under coach Leon Black. Barnes' Longhorns had won their Big 12 opener in each of his first 12 seasons before starting out with a single loss last season.
It keeps getting worse this season with a youthful roster with no upperclassmen in the rotation, and with point guard Myck Kabongo suspended by the NCAA until mid-February.
Oklahoma, meanwhile, is off to a promising start with increasing NCAA tournament hopes.
Osby had the final nine points in an 11-2 run that put Oklahoma up 30-26 at halftime, and the Sooners started to create some distance with a string of seven straight points soon after halftime. M'Baye beat the shot-clock buzzer with a reverse layup and Buddy Hield blocked Demarcus Holland's jumper at the other end before lobbing to M'Baye for a two-handed slam.
Osby followed that by pulling up at the left wing for only his third 3-pointer of the season to extend the lead to 48-36 with 12:25 to play.
"That's not really my role on this team. We have a lot of shooters," said Osby, now 3 for 5 from 3-point range this season. "If it comes in the flow of our offense, then I'll shoot it. But if not, then I'm not worried about making 3s."
Barnes had trouble quantifying how much of a difference it made when Holmes went out, but pointed out that he was Texas' best player with the most experience.
"That's why you have a team," Barnes said. "One thing's for certain now, those young guys going to get a lot of experience. We need them to play, we need them to develop. But it still goes back to we've got to have better guard play to help those post players."