The win gave the Athletics a split of the season-opening series in Japan. Seattle was a 3-1 winner in 11 innings Wednesday.
Oakland had just two hits and a walk off Seattle starter Jason Vargas but touched the Mariners' bullpen for five hits, including the three home runs, in 1 2/3 innings.
Kelley got a fielder's choice but Cespedes followed with a smash into the seats beyond left-center field.
Seattle changed pitchers but Reddick greeted George Sherrill with a towering drive into the right-field seats.
Gomes made the score 4-1 with a two-out home run to left-center in the eighth off Steve Delabar.
Seven of the nine runs in the series came on home runs.