Westbrook recorded 15 points during a third quarter in which Oklahoma City outscored Boston by a 40-24 margin, then put up 10 more in the fourth to help the injury-plagued Thunder to an important win.
Wednesday's result moved Oklahoma City, still without Kevin Durant and now minus Serge Ibaka for likely the remainder of the regular season, a half-game in front of New Orleans for the Western Conference's eighth seed.
Enes Kanter finished with 22 points and 10 rebounds starting in Ibaka's place, while Anthony Morrow gave Oklahoma City a huge lift off the bench by pouring in 20 points on 7-of-11 shooting. Westbrook added 10 assists.
The Celtics, locked in a battle with Indiana and Miami for the East's final two playoff spots after winning a season-high five straight coming in, received a career-high 25 points and nine rebounds from Marcus Smart. Kelly Olynyk and Brandon Bass each tallied 20 points in defeat.