Oakland won its fourth in a row and ended the Padres' three-game winning streak.
Reddick had a two-run triple in the big first inning and Moss added a two-run homer. Cliff Pennington added an RBI double in the fourth and a run-scoring single in the sixth while the A's were scoring twice in both frames.
Travis Blackley, who is playing in the majors this year for the first time in five seasons, made only the 12th start of his career and won for just the second time.
Blackley (1-2) lasted six innings to get the victory, allowing two runs on five hits. His other major league win came in 2004 while with Seattle.
Anthony Bass (2-7) was roughed up for eight runs on eight hits in 5 1/3 innings.