OAKLAND, Calif. -- Marcus Semien made a childhood dream come true with one swing of the bat Monday night at the Oakland Coliseum.

The Oakland shortstop hit his second career grand slam, powering the A's to an 8-5 victory against the San Francisco Giants in the opener of the Bay Bridge Series.

Semien was born in San Francisco, raised in the East Bay city of El Cerrito, went to high school in Berkeley and attended college at the University of California at Berkeley.

"A lot of Giants games, a lot of A's games that my parents and my grandparents took me to as a kid," Semien said. "A lot of these games. To be able to hit a grand slam -- you don't have a lot of grand slams in your career, so for it to be in this series is pretty cool."

The Giants led 3-2 through five innings, but the A's surged ahead 6-3 on Semien's slam in the sixth.

Bruce Maxwell led off the sixth with a walk against reliever Josh Osich and went to third with one out on pinch hitter Rajai Davis' double. Osich (3-2) walked Matt Joyce, loading the bases, and right-hander George Kontos came on to face Semien.

With a 1-1 count, Semien crushed Kontos' belt-high, 92 mph fastball over the left-center-field fence for his fourth home run of the season. It was his first grand slam since April 23, 2014, at Detroit when he played for the Chicago White Sox.

The victory came before a raucous crowd of 38,391, just hours after the A's (47-59) traded ace Sonny Gray to the New York Yankees and veteran infielder Adam Rosales to the Arizona Diamondbacks.

"I think really the distraction of the day was mitigated by the crowd and playing against these guys," said A's manager Bob Melvin, a former Giants catcher. "We don't get too many crowds like that on a Monday night, so when you lose a couple guys like Sonny and Rosie, to be able to go out and have that kind of atmosphere, I think it kind of gets you past that. It certainly motivates you."

Semien went 2-for-5 and scored twice. Jed Lowrie had three hits, including a double, drove in a run and walked once for the A's. Joyce was 2-for-3, walked twice and scored three runs. Ryon Healy had two hits and drove in two runs.

The Giants fell to 40-67 and dropped below the Philadelphia Phillies (39-64) for the worst record in the major leagues. Their pitchers walked nine batters, and four of those baserunners scored.

"That shouldn't happen," San Francisco manager Bruce Bochy said. "Walks killed us. In the major leagues, that shouldn't happen, and that's what did us in."

The Giants cut Oakland's lead to 6-5 with two runs in the seventh, using a double by Carlos Moncrief and singles by Gorkys Hernandez, Denard Span and Brandon Belt. With runners on first and third and two outs, Liam Hendriks came on for the A's to strike out Buster Posey. Hendriks celebrated with a fist pump.

"I'm known to do a little fist pump here and there," Hendriks said. "It's always good ... when you go through a situation like that, you go through and succeed in it. It's definitely an adrenaline rush, and it's something I embrace."

The A's extended their lead to 8-5 in the eighth inning when Healy hit a bases-loaded, two-run single with two outs off Hunter Strickland.

Hendriks pitched a scoreless eighth inning, and Blake Treinen tossed a scoreless ninth for his fourth save of the season and first for the Athletics.

A's rookie Paul Blackburn (2-1) allowed five runs over 6 1/3 innings with four strikeouts and one walk. He had his scheduled start moved up one day when Gray was traded.

"The energy here was great," said Blackburn, a Bay Area native who was born in Antioch and grew up a Giants fan. "I thought for both sides you could hear a bunch of booing and cheering all night."

Giants right-hander Matt Cain allowed two runs on seven hits over five innings in a no-decision. He walked three and struck out one.

"It wasn't always the smoothest innings, but I kind of made it work a little bit," Cain said.

Moncrief went 2-for-5 with a single and a double -- his first two major league hits -- and had his first career RBI. Belt went 2-for-4 with a double, an RBI and a run.

The Giants took a 1-0 lead in the top of the first inning when Belt hit a two-out double to right-center field and scored on Posey's single.

The A's answered with two runs in the bottom of the first. Joyce led off with a walk, moved to second on Semien's single and scored on Lowrie's single. Semien scored when Khris Davis lined a sacrifice fly to left.

San Francisco regained the lead with two runs in the fourth, moving ahead 3-2.

Posey started the rally with a one-out single and moved to third on Brandon Crawford's single. Jae-Gyun Hwang drove in Posey with a single, and Moncrief brought Crawford home with an infield single, his first career major league hit.

NOTES: Oakland traded RHP Sonny Gray on Monday to the New York Yankees for OF Dustin Fowler, INF Jorge Mateo and RHP James Kaprielian, three highly touted prospects. The A's also included $1.5 million international bonus slot cash in the deal. ... Giants backup C Nick Hundley started against Oakland on Monday night after missing two games because of headaches that developed after being hit in his mask by foul tips Friday against the Dodgers. ... The A's traded INF Adam Rosales to the Arizona Diamondbacks for minor league RHP Jeferson Mejia. ... Giants RHP Johnny Cueto (right hand blisters) made a rehab start Monday night with Class A San Jose and allowed no runs on two hits over three innings but was shut down after 34 pitches with forearm tightness. He will be re-evaluated, San Francisco manager Bruce Bochy said. ... A's INF/OF Chad Pinder (strained left hamstring) was reinstated from the disabled list. ... Oakland RHP Kendall Graveman (strained right shoulder) will come off the DL on Thursday to start the series finale against San Francisco.