Athletics 4, Giants 2: Derek Norris blasted a three-run walkoff homer off Santiago Casilla as Oakland rallied to salvage the series finale in dramatic fashion.

Norris hit his first career home run with two outs in the ninth off Casilla (1-3), who was appearing in his third straight game and blew his second save.

It was the second walkoff win in four games for Oakland, which beat the Dodgers on Thursday on Yoenis Cespedes’ homer. Jim Miller (2-0) earned the win with 1 1/3 scoreless innings in relief.

The ending ruined another stellar outing by Giants’ starter Matt Cain, who was looking to win his ninth consecutive start. Cain gave up one run on three hits with seven strikeouts over seven strong innings.

Buster Posey gave the Giants an early lead with his two-run shot in the first off A.J. Griffin, who allowed two runs on three hits over six innings in his major league debut. The Athletics answered in the bottom of the first on Josh Reddick’s RBI single.

Griffin got the start in place of Brandon McCarthy, who was scratched late Saturday with ongoing shoulder soreness.

Oakland had two runners on in the eighth inning, but Jeremy Affeldt induced an inning-ending double-play grounder by Reddick to end the threat.

GAME NOTEBOOK: Oakland’s starting pitchers have allowed four hits or fewer in each of the last eight games, which is the longest such streak in Oakland history. …Giants first baseman Brandon Belt was hitless in three at-bats, ending his career-best 11 game hitting streak. … The Athletics have allowed just 49 hits over their last 10 games. …Griffin was recalled from Triple-A Sacramento, where he was 3-2 with a 2.81 ERA.