Jarrod Parker (9-6) pitched his first career complete game, allowing eight hits. He struck out eight, did not walk a batter and threw 100 pitches.
Josh Donaldson drove in a run for the A's, who have won four of five.
Kendrys Morales drove in a run for the Mariners, who have lost four of six.
Carter Capps (2-3) struck out Yoenis Cespedes to open the ninth before Moss drove a 2-1 pitch to the deepest part of the park.
Mariners starter Aaron Harang gave up a run on five hits over seven innings. He walked one and struck out three.
The A's took a 1-0 lead in the fourth when Josh Reddick and Cespedes singled to open the inning and Donaldson had an RBI grounder.
Morales singled home a run in the seventh to tie it. He then cost the Mariners a chance to take the lead when jogged into third standing up on Justin Smoak's sharp single to right field and was thrown out by Reddick. It was Reddick's 16th assist.
In the eighth, Michael Saunders singled and reached third when Parker threw Dustin Ackley's sacrifice bunt down the right-field line. Parker came back to strike out Humberto Quintero and Nick Franklin around a foul out by Brad Miller.
NOTES: Harang made his major league debut in Oakland over 11 years ago. Mariners interim manager Robby Thompson played in his first World Series game in Oakland in 1989. "It's always special coming back to the Bay Area, especially staying over in San Francisco and walking around," Thompson said. "It brings back a lot of memories, no doubt. I spent a lot of time here. Had some good years here." A's manager Bob Melvin and Thompson were teammates in San Francisco. ... Seattle manager Eric Wedge is coming back from a mild stroke, and is expected to be in uniform on Friday in Seattle. ... Mariners OF Michael Morse missed his second straight game with a sore wrist. ... A's OF Coco Crisp was back in the lineup five days after getting a cortisone shot for his sore left wrist. ... A's LHP Brett Anderson will make his next rehab appearance on Thursday in Class A Stockton.