This three-game series gives the Angels a chance to overtake them.
Los Angeles will try to take advantage of the A's struggles in the opener of this set Friday night.
Oakland (32-22) fell 5-4 to Detroit on Thursday, losing for the sixth time in eight contests as its division lead dwindled from four games to 1 1/2 over second-place Los Angeles (30-23).
A's relievers have compiling a 4.34 ERA in the past seven games, and former closer Jim Johnson was booed off the field again after allowing two runs in the seventh inning Thursday. He has a 6.55 ERA in 22 outings after compiling a 2.94 mark with an AL-best 50 saves for Baltimore last year.
"What am I supposed to do?" Johnson said of being booed. "I don't know what to tell you. Balls are finding holes. I'm throwing pretty good pitches, I just feel like I'm in a little bit of bad luck. I don't think it's as bad as it really seems. I think everybody else thinks that way."
The Angels pounded out 15 hits in Thursday's 7-5 win at Seattle, earning a split of that four-game series.
"There's a lot of weapons in here, one through nine, and guys have their special thing they bring to the team," said right fielder Kole Calhoun, who was 2 for 4 with three runs and two walks from the leadoff spot.
Mike Trout and Erick Aybar combined for six hits and five RBIs against the Mariners.
Trout is batting .417 with nine RBIs during a 10-game hitting streak, while Aybar has a .343 average while driving in nine runs over the past nine games.
Aybar has batted .362 in his past 15 games in Oakland, and Trout collected five hits with two doubles and a home run in the past two overall meetings with the A's.
The Angels, though, hit .227 while dropping two of three to Oakland at home April 14-16.
"We saw them early in the season, they were swinging the bats well then, and they continue to swing the bats well," A's manager Bob Melvin told the team's official website. "Better pitching now, not only in the bullpen but in the rotation."
Oakland's Drew Pomeranz (4-2, 1.38 ERA) will look to rebound from his first defeat since replacing the ineffective Dan Straily in the rotation.
The left-hander allowed two runs with four walks in four innings of a 3-1 loss at Toronto on Sunday. He had won his three previous starts while tossing 15 consecutive scoreless innings.
Pomeranz's only two appearances against the Angels have come in relief, and he's allowed one run in 4 1-3 innings. He tossed 1 2-3 before giving up a walkoff homer to Chris Iannetta in a 5-4, 12-inning defeat April 16.
The Angels' Garrett Richards (4-1, 3.00) has gone 0-2 with a 6.14 ERA in five career starts against the A's. However, the right-hander has been dominant over his last two starts at Oakland, allowing one run in 12 innings while not getting a decision.
Richards is 3-0 with a 1.38 ERA in six road starts this year.
He didn't receive a decision in his most recent outing at home Sunday, yielding three runs with seven strikeouts in seven innings of a 4-3 win over Kansas City.