"Really, it was just a bad game for us overall," A's manager Bob Melvin said. "We've been swinging the bats, we've been playing great defense, we've been getting good pitching. We just didn't play a very good game today."
Posey opened the scoring with a three-run, first-inning homer off A's right-hander Kendall Graveman. Posey's drive to straightaway center field was preceded by a pair of Oakland fielding errors, as shortstop Marcus Semien and second baseman Eric Sogard bobbled grounders. Those miscues made all of the runs unearned.
"That's a situation where I've got to pick up my defense," said Graveman, who finished the spring with a 0.36 ERA in six starts. "They're battling out there for me. Two tough plays. ... It's somewhere I've got to make a pitch to Posey. I can't do that, I can't give that up. That's on me. I've got to find a way to get another ground ball."
Giants starter Matt Cain blanked Oakland on three hits through four innings. An elevated pitch count of 79 hastened Cain's removal. The right-hander made his first appearance at AT&T Park since his final outing of 2014 -- a six-inning Interleague victory over Oakland last July 9. About a month later, Cain underwent surgery to remove bone chips and spurs from his throwing elbow.
"That felt really good to get back in this whole atmosphere," Cain said.
Graveman also worked four innings, yielding two hits and striking out six. After pitching five games in relief for Toronto last season in his Major League debut, he will open the season as Oakland's No. 4 starter.
"It'll be my first start in the big leagues, something I'm looking forward to," Graveman said. "Something you look forward to your whole life. I'm definitely ready to pitch in front of our home crowd and get out there and compete once again."
The Giants added a run in the sixth off A's left-hander Eric O'Flaherty. Brandon Belt drilled a leadoff single and advanced to second base one out later on Justin Maxwell's infield hit. O'Flaherty's wild pitch advanced the runners before Joaquin Arias' infield single scored Belt.
A's Up Next: The A's will wrap up their exhibition schedule with a 1:05 p.m. PT matchup against the Giants on Saturday at O.co Coliseum. Southpaw Drew Pomeranz, who was named as Oakland's fifth starter by A's manager Bob Melvin on Thursday, will get the start in the Bay Bridge Series finale. Melvin said he also plans on using Barry Zito in relief, and the veteran figures to get a warm welcome in his first home game with the green and gold since 2006.
Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, and follow him on Twitter at @sfgiantsbeat. This story was not subject to the approval of Major League Baseball or its clubs.