OAKLAND, Calif. — Jemile Weeks hit a go-ahead RBI single in the seventh following Coco Crisp’s leadoff triple, and the Oakland Athletics completed a three-game sweep of the Boston Red Sox with a 3-2 win Wednesday.

Red Sox slugger David Ortiz hit his 400th career home run and also scored on Adrian Gonzalez’s sixth-inning single, but Boston couldn’t avoid its first sweep by the A’s since May 23-25, 2008, also in Oakland. The Red Sox lost just their fourth series in the last 17 and completed a disappointing 2-5 road trip through Seattle and Oakland, the AL West’s two bottom teams.

Brandon Moss homered leading off the second for his 10th of the season, giving the A’s longballs in 15 straight games — most for the club since doing so in 17 in a row from April 12-May 1, 2002.

Moss singled in the fourth and doubled in the sixth, leaving him a triple shy of the cycle.

Brandon Inge doubled home Moss to tie the game at 2 in the sixth.

Grant Balfour (2-2) went 1 2-3 innings for the win as the A’s (41-42) moved within one game of .500 for the first time since they were 22-23 on May 23. Lone A’s All-Star Ryan Cook finished with a 1-2-3 ninth for his eighth save in 11 chances.

Ortiz hit his 400th homer with a leadoff drive to right in the fourth inning against Oakland starter A.J. Griffin.

The 36-year-old Big Papi’s home runs rank eighth among active players and 49th on the all-time list. Next up for him to catch: Hall of Famer Duke Snider with 407.

Ortiz’s 22nd home run of the season tied the game at 1. Ortiz has 352 home runs and 1,142 RBIs as a designated hitter for most all-time by a DH.

Ortiz tipped his batting helmet as his milestone was announced to a standing ovation before he stepped in to hit in the sixth, then drew a five-pitch walk.

Aaron Cook (2-2), coming off a career-best two-hit shutout in a 5-0 win at Seattle on Friday in which he didn’t walk a batter for the second straight outing, was tagged for nine hits and three runs in six-plus innings.

Facing Oakland for the first time since June 28, 2009, while with Colorado, the right-hander fell to 4-2 in six career starts versus the AL West.

Griffin is still looking for his first major league win after six solid innings.

Second baseman Weeks’ fielding error in the sixth led to Boston’s unearned run and cost Griffin a shot. Gonzalez’s RBI single extended his season-best hitting streak to 15 games, longest active in the majors. He had 10 hits during the seven-game road trip to Seattle and Oakland.

Red Sox Hall of Famers Ted Williams and Carl Yastrzemski also hit their 400th home runs against the Athletics franchise. Williams hit his against the Kansas City A’s.

The A’s also allowed Paul Konerko’s milestone 400th home run for the White Sox when he did it April 25 against Grant Balfour in the Coliseum.

Notes: The clubs met for the 30th time on July 4. … Oakland leads the season series 5-1. …  This marked Boston’s first road series loss since dropping two of three at Kansas City from May 7-9. … The Red Sox selected RHP Justin Germano from Triple-A Pawtucket. … Red Sox 3B Will Middlebrooks missed his third straight game with a left hamstring injury that forced him out of Sunday’s game at Seattle after the top of the ninth because of tightness. Manager Bobby Valentine hopes to keep Middlebrooks off the DL considering he wouldn’t be available to return until after the All-Star break.