The Thunder had sat atop the West for most of the season but an 11-game winning streak from surging San Antonio put the Spurs percentage points in front heading into the night.
The teams flipped positions after Oklahoma City took care of their business in Milwaukee and the Spurs later lost at Utah Jazz after deciding to rest key veterans Tim Duncan, Tony Parker and Manu Ginobili.
Oklahoma City (42-15) cruised behind 26 points from Russell Westbrook and 19 from Kevin Durant as they raced out to a 35-18 first-quarter lead which they never looked like relinquishing.
"That's what you're supposed to do," Westbrook said. "If you want to be able to win championships in this league you've got to be able to come out every night, regardless if it's home or on the road, to impose your will on teams."
The Bucks (28-29) are trying to close in on the last play-off spot in the East but fell 1.5 games behind eighth place after a frustrating night where they earned five technical fouls, including two for forward Larry Sanders who was ejected.
The game featured some tense moments including a first-half confrontation between Mike Dunleavy and the Thunder's Serge Ibaka, who grabbed Dunleavy's arm as the two went face-to-face and were both assessed technicals.
Milwaukee could not turn their passion into production, however, and they were outmatched in every department.
Ersan Ilyasova led the way with 18 points but the home team got little out of trade acquisition Monta Ellis, who made just three of 12 shots for nine points.
The Thunder defense had a lot to do with that as Thabo Sefolosha recorded seven steals and Ibaka added five blocks to help their team to a second straight win after a three-game skid.
"(Ibaka) protects the basket so well," said Durant. "It makes it easier for our defense. Sometimes when we get broken down on the wing, Serge is there to help us out."