The Sooners were just 1 of 6 from the floor in the final three minutes and were outrebounded 10-2.
“It got physical,” Spangler said. “We weren't hitting shots that we usually make outside.”
The Sooners' late struggles wasted another excellent game from Spangler.
He had a career-high 21 points and 14 rebounds. Nine of his rebounds came at the offensive end.
In the last two games, Spangler has 37 points and 29 rebounds.
Clark, who came in leading the Big 12 in scoring with 18.3 points per game, scored his only basket with about eight minutes remaining.
Hield scored eight points in the first six minutes but had just four more the rest of the way.
The duo was a combined 6 of 25 from the floor as the Sooners had their worst shooting night of the season. Oklahoma was just 22 of 66 (33.3 percent) from the floor and just 7 of 23 from behind the 3-point line.
“I just couldn't get going man,” Clark said, shaking his head. “I couldn't get going.”
Clark said he wasn't worried about bouncing back, but said he had to adjust to some of the defensive looks he was seeing this season.
“I'm about to go to the gym and just work on my craft — get better,” Clark said. “I'm not really worried about the scoring. I didn't do what I needed to do personally. I feel like I could've helped my team out more on the defensive end, just rebounding. I've got to work on more things if my shot's not falling.”