The A's added six runs off Giants closer Sergio Romo in the fourth inning while amassing seven hits, including Yoenis Cespedes' RBI single, Brandon Moss' two-run single, Stephen Vogt's two-run triple and Michael Taylor's run-scoring single.
Oakland's fielding helped preserve that lead. Second baseman Eric Sogard began an inning-ending double play in the first by gobbling up Pablo Sandoval's grounder and flipping the ball from his glove to shortstop Nick Punto, who crossed the bag with balletic timing.
One inning later, Reddick reached over the top of the 10-foot-high right-field fence to rob Michael Morse of an opposite-field home run.
"I climbed it, and as soon as I turned my head, the ball was pretty much over the fence, and I just quick-snatched it, I guess you could say," said Reddick. "I was shocked, even being able to come close to it."
Reddick repeated the feat with two runners aboard in the fourth inning, reaching to the top of the barrier to deny Morse an extra-base hit, if not a homer.
"He's known for doing that, man," Morse said. "I'm happy to help him work on it in Spring Training, I guess."
Earlier in the fourth, Sandoval's sacrifice fly drove in the Giants' first run. Adam Duvall hit a leadoff homer in the ninth and Ehire Adrianza belted a three-run shot with two outs.
Up next: It's rotation hopeful Tommy Milone taking the mound for the A's in their home opener against the Brewers at Phoenix Municipal Stadium at 12:05 p.m. PT on Thursday. The lefty won 25 games for the A's over the previous two seasons, but he currently appears to be the odd man out in a rotation that already boasts Jarrod Parker, Sonny Gray, Scott Kazmir, A.J. Griffin and Dan Straily. Still, a strong spring performance could keep him in the mix.
Milone will have plenty regulars playing behind him, with Coco Crisp, Josh Donaldson, Jed Lowrie, Cespedes, Reddick, Derek Norris and Sogard all slated to start the game.