The pitchers' duel was decided on a one-out, 2-1 fastball from Capps (2-3) that Moss sent to straightaway center for his 19th home run of the year.
"Watched it about four or five times already and I felt like I executed the pitch pretty good," said Capps. "I was trying to throw a strike low and away and I did, he just got to it."
The blast secured a win for Jarrod Parker (9-6), who tossed a complete game for the first time and stayed unbeaten in his last 15 starts. Parker scattered eight hits and allowed one run while striking out eight with no walks.
"He's the guy that's pitching the best right now. We feel great about him," A's manager Bob Melvin said. "Not only is he throwing the ball well, he's getting quick outs, which is allowing him to pitch deep in games."
Seattle starter Aaron Harang did not factor in the decision despite holding the A's to one run on five hits over seven innings.
Oakland has won four of its last five while Seattle fell to 3-4 on its nine- game road trip.
Josh Reddick and Yoenis Cespedes each singled to open the home fourth inning, and Reddick scored when Josh Donaldson beat out a potential inning-ending double play ball.
Parker cruised through the first six innings before Nick Franklin opened the seventh with a single and moved to second when Reddick bobbled the ball in right field. Kendrys Morales singled in Franklin two batters later to tie the game, but Reddick made up for his previous miscue by throwing out Morales at third later in the inning.
A error by Parker put runners on the corners with no one out in the eighth, but the right-hander escaped with the game tied with two strikeouts around a pop fly in foul territory.
Parker, who threw 100 pitches, has won his last seven decisions since last losing to the Rangers on May 22 ... The A's have a 2.19 ERA over the last seven games ... The A's have committed 17 errors in their last 18 games ... Harang is winless in his last five starts ... Seattle still leads the season series, 8-6 ... Game time: 2:19.
08/20 02:45:36 ET