Canseco, last year's minor league player of the year, had just three hits in 19 at-bats this spring when he blasted a pitch from loser Juan Berenguer 420 feet over the center-field fence to give the A's a 5-1 lead in the third inning.
In the sixth, with Atlee Hammaker on the mound, the Cuban-born Canseco hit his mammoth drive. It cleared a row of trees beyond the 390-foot mark in the gap in left-center field.
Oakland scored three runs in the first inning on its way to making the Giants losers for the first time in eight games this spring.
Giant rookie first baseman Will Clark hit his fourth home run this spring and his third against the A's. Chris Brown also homered for the Giants.