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: Following Thursday's scheduled off-day, the Giants will get back to work in earnest Friday, with 12:05 p.m. PT split-squad games at Oakland and against Milwaukee at Scottsdale Stadium. Catcher Buster Posey and center fielder Angel Pagan are expected to make their first Cactus League appearances. Opening Day starter Madison Bumgarner will face Milwaukee, while Edwin Escobar, one of the Giants' brightest pitching prospects, confronts Oakland.
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.