OAKLAND, Calif. -- Derek Norris will have a hard time topping his first major league home run celebration.

Norris hit a three-run shot with two outs in the ninth inning for his first career home run, and the Oakland Athletics walked off with a stunning 4-2 victory over the San Francisco Giants on Sunday.

"It's something I'll never forget," Norris said.

Norris hammered a full-count pitch from Santiago Casilla (1-3) over the left-field wall to help the A's avoid the first three-game sweep by San Francisco in Oakland. A's players streamed out of the dugout in those bright gold uniforms to mob Norris at home plate and turn the infield into one big blur.

The new A's catcher flicked off his helmet and spread his arms out wide before touching home. He later took a pie to the face, then water and a sports drink in succession from teammates in a chaotic cluster that left a mound-size stain on the dirt near Oakland's dugout.

Almost as big as the mark Norris made in his Bay Bridge Series introduction.

"The pie not so good," Norris said, laughing. "My eyes are still trying to come back. Actually, I can only see about half of you right now. It's a good feeling. I wouldn't trade it for the world."

The homer handed Casilla his second blown save in 22 chances this season in place of bearded closer Brian Wilson. The ending also washed away San Francisco starter Matt Cain's bid for a ninth straight win.

"Not now," Casilla said, declining to speak to reporters.

Jim Miller (2-0) pitched 1 1/3 hitless innings for the victory.

Buster Posey's two-run homer in the first was all the Giants could squeeze out against A.J. Griffin, who retired 16 of the next 18 batters he faced in his major league debut. Griffin gave up three hits, one walk and struck out four in six-plus innings.

Not bad for a guy A's manager Bob Melvin didn't even recognize when he showed up in his office from Triple-A Sacramento before the game.

"That's got to be the thrill of a lifetime," Griffin said after his former minor league teammate's homer. "You can't expect to have your first home run a walkoff, let alone against the rival Giants from across the Bay."

Or match a pitcher only two starts removed from a perfect game.

Cain allowed three hits and two walks, and struck out seven in seven innings. He was spared a potential two-run double by Seth Smith in the sixth inning that was called foul. Replays appeared to show that the ball landed fair.

Oakland overcame all that and more.

Jeremy Affeldt wiggled out of a jam in the eighth, pumping his fist and kicking his legs after getting Josh Reddick to ground into a double play. Casilla seemed ready to do the same in the ninth until Norris ended those hopes with one swing.

The rally started with Yoenis Cespedes' leadoff single. Casilla struck out Smith, but Brandon Inge also singled before Casilla struck out Brandon Moss.

On the seventh pitch of the at-bat, Norris hit a towering drive over the high wall in left. He put his hands in the air as the A's turned the infield into a gathering of gold.

What a dramatic way for the Bay Bridge Series to end.

San Francisco scored four runs in the ninth to rally for a 5-4 victory on Friday night. Oakland almost pulled off an even more impressive comeback Saturday, scoring five runs in the ninth before falling just short in a 9-8 loss.

"I knew it was out," Giants manager Bruce Bochy said. "This one they got to walk off. You live with the ups and downs of the game. We got the first two."

Both starting pitchers provided some stability to a series that had little of it for most of the weekend.

Griffin, a 24-year-old player from the San Diego area selected by the A's in the 13th round of the 2010 draft, retired 13 straight batters before Posey walked to open the seventh. Griffin trotted off the mound to a roaring ovation from a rare sellout of 36,067 fans at Oakland Coliseum.

About the only other thing that had the home fans riled up before the finish was a contested call.

With two outs and runners on first and second, Smith lined a full-count pitch by Cain down the right-field line that first base umpire Todd Tichenor called foul. Smith put his hands on his helmet and shook his head in disgust, and Melvin came running out of the dugout to protest.

A zoomed-in replay appeared to show the ball land on the chalk, which would have likely given the Athletics a 3-2 lead. Instead, after Melvin argued for several minutes, Smith fouled out to left on the 12th pitch of the at-bat.

Notes

Oakland placed opening day starter Brandon McCarthy (shoulder strain) on the 15-day disabled list, retroactive to Wednesday, called up reliever Evan Scribner, and optioned infielder Eric Sogard to Triple-A Sacramento before the game. ... Hector Sanchez will catch LHP Barry Zito (5-5, 4.35 ERA) on Monday when the Giants face RHP Nathan Eovaldi (0-3, 2.35 ERA) and the Los Angeles Dodgers in San Francisco. Giants manager Bruce Bochy had flirted with the idea of keeping Posey behind the plate instead of Sanchez, who has caught Zito most of the season.