Seattle scored a run in each of the first two innings but the A's struck back for four in the third and coasted from there.
After clearing the bases with his double in the third, Matsui scored on an infield single by Brandon Allen.
Oakland had another four-run inning in the seventh, during which Cliff Pennington had a three-run homer.
Guillermo Moscoso (7-8) settled down after allowing the two early runs and picked up the victory by giving up just five hits in seven innings. He struck out seven.
Jason Vargas (7-13) suffered through the big third inning and eventually coughed up five runs on six hits in five frames to take the loss.
Swim Week Miami Beach 2014 [PHOTOS]