For the fourth straight game, the Western Conference-leading Thunder won after trailing by at least a dozen points.
Thunder forward Luguentz Dort scored 11 of his 14 points in the fourth quarter, drilling back-to-back three-pointers to put Oklahoma City up 100-88 with 1:59 remaining.
Dort drained another from beyond the arc to cap the scoring in the final minute.
In a game tipped as a potential NBA Finals preview, the Thunder held the Celtics to just 27 points in the second half, snapping Boston's winning streak at three games.
"We're out there having fun," Dort said. "Anytime somebody does something well, we're always out there cheering for them. We've been building this chemistry for years, and we're clicking right now."
The game was the second of three straight against the Eastern Conference's top three teams for the Thunder.
On Friday, they rallied to end the third-place Knicks' nine-game winning streak.
With the second-place Celtics taken care of, the 30-5 Thunder will look forward to taking on the Eastern Conference-leading Cavaliers in Cleveland on Wednesday.
The Cavs hosted the Charlotte Hornets later Sunday aiming to improve their 30-4 record.
Jayson Tatum scored 26 points and grabbed 10 rebounds for the Celtics. Jaylen Brown scored 21 points and Kristaps Porzingis added 19, but Boston connected on just 9-of-46 three-point attempts and, like a string of teams before them, crumbled in the face of Oklahoma City's suffocating second-half defense.
Brown, back after missing two games with a shoulder injury, piled up 10 points in the first period to help the Celtics emerge from a tight first quarter with a three-point lead.
Boston pushed the advantage to as many as 13 before hitting halftime with a 10-point edge.
Oklahoma City whittled that to four going into the final quarter and regained the lead for the first time since the first quarter when Jalen Williams drilled a pair of free-throws minutes into the fourth.
The Thunder wouldn't trail again. Although the Celtics twice pulled level, they had no real answer.
Tatum's nine points were the only points by a Celtics starter in the final frame and Brown was held scoreless on 0-for-7 shooting in the second half.
