Portland, OR – Russell Westbrook blocked Nicolas Batum's ahead 107.
Prior to Westbrook's block, LaMarcus Aldridge was whistled for a controversial goaltending violation on Kevin Durant's layup attempt, tying the game with six seconds remaining.
Portland shot just 1-of-9 from the field in the extra session to fall to 11-2 at Rose Garden this season.
Durant finished with 33 points, Westbrook added 28 points, 11 rebounds and eight assists, and Serge Ibaka chipped in 12 points and 13 boards for Oklahoma City, which was coming off Saturday's loss at San Antonio.
Aldridge led the Blazers with 39 points, but was called for goaltending despite appearing to get a piece of Durant's scoop before it hit the backboard.
Batum drove to the basket with time winding down, only to be met in the lane by Westbrook, who was able to hang in the air long enough to get a piece of the ball and send the game to overtime.
Westbrook opened the OT scoring with a pullup jumper, and Harden added a dunk off a Jamal Crawford turnover with 2:53 on the clock.
A Crawford basket cut the deficit to two, but Batum and Crawford each missed shots in the final minute, the latter getting blocked by Kendrick Perkins with under 10 seconds to play.
"[Perkins] was the difference maker. He made them work for every shot," Thunder head coach Scott Brooks said.
Durant slammed the door with a dunk.
Reggie Jackson's three-pointer near the first-quarter buzzer pulled Oklahoma City within 31-29, and the Thunder scored the final eight points of the half to build a 60-52 advantage.
The difference moved to 12 early in the third, but Portland responded with the next 13 points to continue a back-and-forth affair that featured 13 ties and just as many lead changes.
Oklahoma City owned an 85-79 edge heading to the fourth, and it was a two- possession game the entire final stanza.
The Thunder committed 20 turnovers, but scored 23 points off 13 Portland giveaways...Wesley Matthews and Crawford totaled 18 and 17 points, respectively in defeat...Oklahoma City had 20 fastbreak points to the Blazers' seven in transition.