And when the two teams met for the first time this season, they continued to head in opposite directions.
Jesse Chavez matched a career high with nine strikeouts in six innings, and three relievers pitched scoreless ball to lead the A's past the Giants 5-0 on Monday night for their fifth straight win.
"It's exciting to be part of the Oakland Athletics right now," said third baseman Josh Donaldson, one of six A's players named to the American League All-Star team.
The A's have been among baseball's best and most consistent clubs all season. Then they made a surprising trade to get right-handers Jeff Samardzija and Jason Hammel from the Chicago Cubs on Friday for three minor leaguers, including top prospect Addison Russell.
Oakland also leads the majors with its All-Star selections, the club's most since 1975. Sunday's announcement didn't even include Samardzija, who was selected as a National League All-Star but is ineligible to play.
The banged-up Giants, on the other hand, had lost 18 of 23 to fall out of first place in the NL West -- where they once held a 9 1/2-game lead over the Dodgers -- but showed signs of coming out of their slump by winning two in a row at San Diego over the weekend.
Instead, the two-time defending AL West champion A's put together the kind of all-around effort they've had most of the season in the Bay Bridge Series opener.
Alberto Callaspo hit a two-run double, and three others drove in a run to power the A's in front of an announced sellout crowd of 36,067 festive fans at the Coliseum. The worn-down stadium had a colorful sprinkling of green and gold from Oakland revelers mixed with the orange and black of San Francisco supporters -- many of whom left before the final out.
Chavez (7-5) allowed four hits and two walks to help extend the A's record to a major league-best 56-33. Fernando Abad, Dan Otero and Ryan Cook shut down San Francisco the final three innings.
"We all like to feed off each other," Chavez said. "We're just playing good baseball, and we're excited to be back home."
Ryan Vogelsong (5-6) lost his third straight start for the Giants, who had just five hits.
The right-hander gave up five hits and three runs in 5 1/3 innings. He struck out four, walked two and hit a pair of batters, leading to the A's first two runs.
Oakland started its scoring surge after plate umpire Paul Nauert ruled Craig Gentry was hit by a pitch on his hand leading off the fifth. Giants manager Bruce Bochy challenged the play, which was upheld after a video review.
Gentry stole second, advanced to third on Coco Crisp's bunt single and scored on John Jaso's groundout for the A's first run.
Donaldson also was hit by a pitch by Vogelsong on his left hand leading off the sixth. Nauert originally called it a strike before seeing Donaldson's hand swell.
Then Nauert asked first-base umpire Angel Hernandez whether Donaldson, who said his hand was just sore afterward, swung at the pitch. Hernandez signaled no, and Bochy came out and had a heated conversation with Nauert after the call, wondering how it could be reversed.
"Then it got ugly," Bochy said.
Callaspo came through later in the inning with a two-run double off Juan Gutierrez to extend Oakland's lead to 3-0. Brandon Moss and Donaldson each drove in a run off Gutierrez in the seventh after Giants shortstop Brandon Crawford committed two throwing errors.
The teams will play in Oakland again Tuesday night before the series shifts to San Francisco for the final two games.
Giants 3B Pablo Sandoval, who left Saturday's game because of a bruised left elbow after he was hit while swinging at a pitch, sat out for the second straight game. ... Athletics RF Josh Reddick took batting practice and fielded fly balls for the first time since reaggravating his right knee injury June 28 at Miami. ... Giants 2B Marco Scutaro (back) went 1-for-3 with an RBI single in his first rehab assignment for Triple-A Fresno on Monday night at Sacramento. ... Giants CF Angel Pagan was sent to Arizona to resume his rehabilitation from a nagging back injury. ... Giants lefty Madison Bumgarner (9-6, 3.09 ERA) takes the mound opposite Oakland's Sonny Gray (8-3, 3.08) on Tuesday.
Chavez Guides Athletics To Fifth Straight Win
Jesse Chavez matched a career high with nine strikeouts as the Athletics beat the Giants 5-0.