BOSTON - The young Oklahoma City Thunder needed to beat just two more teams to reach their goal: wins against every club in their two seasons since moving from Seattle.

And not just any ordinary teams.

On Wednesday night, the Thunder completed the feat by edging the 2008 NBA champion Boston Celtics 109-104. Just five days earlier they beat the other, the Los Angeles Lakers — last season’s champions.

“We’ve grown up,” said Kevin Durant, who scored 37 points and hit all 15 of his free throws against Boston. “It means a lot for us. It gives us confidence.”

Durant and Jeff Green, who hit two big 3-pointers in the last two minutes, were rookies in the last season in Seattle. Point guard Russell Westbrook, who had 21 points and 10 assists, joined them last season, and guard James Harden, who is averaging 9.9 points per game, is contributing as a rookie.

And now they’ve beaten the last two champs in a six-day span.

“Going into the week that was our battle cry,” coach Scott Brooks said, “and both in the same week.”

Boston is among the NBA’s oldest teams. The Celtics’ starting five has an average age of 29.6, while the average for Oklahoma City’s starters is 23.2.

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The Thunder won their 46th game, doubling their total from last season, and remained in sixth place in the Western Conference.

Boston was led by Kevin Garnett and Rasheed Wallace with 18 points each and fell out of a third-place tie in the East with the Atlanta Hawks, who beat the Lakers 109-92.

Both teams played very well offensively, and Garnett said the free-throw discrepancy may have been the difference. The Thunder outscored the Celtics 28-13 at the line, even though Boston was whistled for just four more fouls than Oklahoma City.

Garnett was particularly concerned about Durant’s 15 free throws.

“I thought we were playing Michael (expletive) Jordan the way he was getting the whistle,” Garnett said. “Durant damn near shot more free throws than our whole team.”

Boston went 13 for 17 from the line.

“I missed three last night in Philly” in a 111-93 Thunder win over the Philadelphia 76ers, Durant said. “I was a little upset with myself.”

Green, whose draft rights were traded by the Celtics when they picked up Ray Allen from Seattle after the 2006-07 season, gave the Thunder a 105-101 lead with a 3-pointer with 1:56 to go. Then he connected again with 1:22 left for a 108-104 advantage to finish with 17 points.

“It’s just me staying with my rhythm,” Green said. “Nenad (Krstic) set some great screens on (Garnett).”

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The Celtics were having one of their best shooting nights of the season and Wallace had made both his 3-point attempts. But then he missed with 1:11 to play, keeping the Thunder’s lead at 108-104. Four more misses followed — a jumper by Durant, a layup by Boston’s Paul Pierce, a 3-pointer by Westbrook, and a jumper by Allen with 9.1 seconds to play.

Thabo Sefolosha finished the scoring with a free throw with 6.9 seconds remaining.

The Celtics lost even though they made 59.5 percent of their shots.

“It’s a tough one to swallow. A team shoots 34 free throws to our 17 on our home floor,” Boston coach Doc Rivers said. “I thought we were attacking and we just didn’t get to the line.”

Despite the loss, it was quite a change from the Celtics’ last game, a 94-73 loss to the San Antonio Spurs on Sunday night in which Boston lost by its biggest margin and scored its fewest points of the season.

“The game the other night we played so bad. It happens,” Rivers said. “It’s tough when you play well and lose.”

On Wednesday, the Celtics led 61-57 after a tight first half in which the lead changed hands 16 times. The margin was just four points despite one of their best offensive quarters of the season. In the second period, the Celtics hit 17 of 20 shots (85 percent) and tied their season high for a quarter with 38 points.

Boston made its first two shots of the third quarter, boosting its marksmanship to 29 of 40, a 72.5 percent clip, but couldn’t pull away.

Notes: Durant scored at least 30 points for the 40th time in 74 games. ... Boston C Kendrick Perkins returned after missing two games with tendinitis in his left knee. ... Pierce had 15 points in 37 minutes after aggravating a “stinger” in his right shoulder during practice Tuesday. ... The Thunder capitalized on nine Celtics turnovers to score 10 of their 30 points in the first quarter.

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