Seattle's Michael Morse extended his hot start, cracking his fourth home run just three games into the season in the sixth inning but it was far short of countering Reddick.
Reddick hit his first home run of the season in the first inning, doubled in a run in a sixth and added an eighth-inning sacrifice fly in leading the 10-hit Oakland attack.
Oakland starter A.J. Griffin (1-0) benefited from the barrage, giving up seven hits and two runs while striking out three and walking one. In addition to Morse's home run, Griffin allowed a fifth-inning sacrifice fly by Michael Saunders.
John Jaso followed that, however, with an RBI single in the bottom of the fifth and, after Morse made it a 3-2 game, Reddick doubled in Jed Lowrie and scored on a home run by Yoenis Cespedes in the sixth.
Reddick and Nate Freiman each had sacrifice flies in the eighth inning, completing the scoring.
Oakland roughed up Mariners starter Brandon Maurer (0-1) for six runs on eight hits over six innings.