OAKLAND (AP) — Madison Bumgarner had a no-hitter through three erratic innings, and then one pitch turned a grinding start into a loss.

Bumgarner matched a career high with five walks and gave up a two-run homer to Josh Donaldson, and the San Francisco Giants fell 4-1 to the Oakland Athletics in the Bay Bridge Series opener Monday.

“I wasn’t missing by a big margin,” Bumgarner said. “Had a lot of walks and that cost a couple runs. Tried to battle through. The biggest thing today was I wanted to try to go deep and give the bullpen a rest.”

Donaldson connected against Bumgarner (4-3) in the fourth, and Yoenis Cespedes added a two-run double against George Kontos in the seventh to extend Oakland’s lead.

Bumgarner struck out six and was charged with four runs in 6 2-3 innings. He has lost three of his last four starts, though manager Bruce Bochy felt good about the left-hander’s latest outing.

“Our offense has to get it going here a little bit,” Bochy said. “That’s the kind of pitching we’re going to need. It doesn’t go down as a quality start, but he left the game with a really good start and that’s what we’re going to need playing these good clubs. He did all he could. Their guy pitched well.”

Buster Posey’s groundout in the sixth off Dan Straily (3-2) drove in San Francisco’s lone run. Straily allowed four hits and one walk in six innings.

Sean Doolittle pitched two perfect innings and Grant Balfour recorded his 11th save of the season for Oakland, which has won four straight and nine of 10.

“I didn’t watch much of the playoffs last year, but I heard the Giants won the World Series,” Doolitte said, sarcastically. “So they’re the defending champs in your building. That’s a crosstown rival. That’s a game you get up for. Packed house. The crowd was electric. It was an awesome environment.”

The usual sprinkling of green-and-gold clad fans mixed with San Francisco’s orange-and-black following that traveled from across the bay to pack the Oakland Coliseum. The announced sellout crowd of 36,067 traded chants of “Let’s go Oakland!” and “Let’s go Giants!” throughout the cloudy afternoon.

Straily retired 14 of his first 15 batters and looked confident and in control from the start. Bumgarner struggled with his command but didn’t allow a hit until the fourth.

That hit turned out to be the difference.

Donaldson sent a cutting 89 mph fastball over the right-center field wall to give the A’s a 2-0 lead. Donaldson raised his right hand in the air after jogging around first base, celebrating his eighth home run with a little extra flair.

“Just trying to throw fastball down and away,” Bumgarner said. “It kind of came back over the middle a little bit, but it was down. He was looking to go the other way and put a pretty good swing on it.”

San Francisco squandered its best chance for a big inning when Posey grounded out in the sixth to score Gregor Blanco, snapping Straily’s scoreless streak at 12 1-3 innings. Then Straily snared Hunter Pence’s grounder to strand Marco Scutaro on third.

Oakland’s bullpen backed up his outing far better than San Francisco’s did for Bumgarner, who was relieved after walking Chris Young with two outs in the seventh. Cespedes followed with a towering double off the right-center field wall that gave Oakland a 4-1 lead.

“Today was tough. We didn’t really get anything going but for the most part we’ve been hitting well,” Giants shortstop Brandon Crawford said. “We aren’t too worried about our offense right now.”

NOTES: Giants reliever Santiago Casilla said he will have surgery Tuesday to remove a cyst from inside a bone in his right leg. He hopes to return just after the All-Star game on July 16. ... Giants CF Angel Pagan (sore left hamstring) sat out for the second straight day but hopes to be back in the lineup Tuesday. ... The Giants optioned RHP Sandy Rosario to Triple-A Fresno after the game to make room for lefty Michael Kickham, who will make his major league debut on Wednesday against Oakland’s Jarrod Parker (2-6, 5.76 ERA). Kickham takes over the spot in the rotation vacated by Ryan Vogelsong, who fractured his right hand when he was hit by a pitch May 20.