OAKLAND, Calif. -- Brandon Moss might just make Boston miss him yet.
He finished 6 for 8 with five RBI and three runs scored against his former club that traded him away.
"Playing against a team you came up with and team you played your first little bit in the big leagues is special," Moss said. "You want to do well, not out of animosity, but to show, 'Hey, you did draft a good player.' "
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Jemile Weeks hit a go-ahead RBI single in the seventh following Coco Crisp's leadoff triple in Oakland's fourth consecutive victory.
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 only their fourth series in the past 17 and completed a disappointing 2-5 road trip through Seattle and Oakland, the AL West's two bottom teams.
"I wish he could celebrate," manager Bobby Valentine said. "David is a team guy and he knows it's a great individual accomplishment, we all know that. But it's tarnished."
Still, Ortiz returned to the clubhouse to some 40 congratulatory messages.
"And my phone is still blowing up," he said. "I know at some point in my career when I'm not playing baseball I might look at it from the outside and be like, 'Whoa, I guess I had a good career.' I look at it right now as just another home run."
Moss hit a home run leading off the second for his 10th of the season, giving the A's longballs in 15 consecutive 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 home run 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 career list. Next up for him to catch: Hall of Famer Duke Snider with 407.
"I was trying to attack the zone. Papi got me. Congrats to him," Griffin said. "That's a great milestone right there. My name will be next to that one."
Ortiz's 22nd home run of the season tied the game at 1. Ortiz has 352 home runs and 1,142 RBI 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. He was thrilled with the surprising gesture from a crowd of 28,240.
"That was pretty cool. You're playing on the road and you get something like that on the road, where people really appreciate it," he said.
Aaron Cook (2-2), coming off a career-best two-hit shutout in a 5-0 victory at Seattle on Friday in which he didn't walk a batter for the second successive 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 victory 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.
"Our offense has just been terrible," Boston right fielder Cody Ross said. "I don't need to sugarcoat it. It just [stunk] basically. Our pitchers have been pitching their butts off to keep us in games and we just can't seem to push a run across."
Red Sox Hall of Famers Ted Williams and Carl Yastrzemski also hit their 400th home runs against the Athletics franchise -- both also doing so in July. Williams hit his against the Kansas City A's.
- The clubs met for the 30th time on July 4.
- Oakland leads the season series 5-1.
- Boston 2B Dustin Pedroia was held out to rest his troublesome right thumb, which he hurt again Tuesday night diving for a ball in right field. He will be evaluated back in Boston on Thursday.
- 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 consecutive 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, but he expects to return this weekend. Valentine hopes to keep Middlebrooks off the DL considering he wouldn't be available to return until after the All-Star break.
- Ten men and women were sworn into the U.S. Marine Corps on the field before the game.