The Angels lost, too, keeping them 1 1/2 games back of Oakland for first place in the American League West.
But they're not going anywhere. That's why the A's braved a blockbuster and reeled in Jeff Samardzija and Jason Hammel from the Cubs two weeks ago.
Hammel, though, didn't offer much to revel over in his home debut for the A's, getting just six outs and allowing five runs before handing the ball over to a bullpen that surrendered three more.
The veteran righty, facing the O's for the first time since pitching for them from 2012-13, didn't even record one out before allowing a three-run homer to Adam Jones, who finished the night with a career-high-tying five RBIs.
Leadoff man Nick Markakis went 4-for-5, and No. 2 hitter Steve Pearce contributed a pair of doubles, giving Baltimore's first three hitters nine of the club's 13 hits.
The A's as a team had eight.
"I played with those guys for two years, so they know me pretty well, I know them," said Hammel, who gave up a second first-inning long ball to J.J. Hardy. "Just bad execution on my part. Bottom line is I have to do better.
"You like to show them what they're missing, but they got the best of me tonight. There was outstanding energy out there tonight with the celebration, a lot of fun, so it's disappointing to send them home with an L."
Chris Davis' RBI double in the third marked the end for Hammel, who has allowed eight runs (seven earned) in seven-plus innings spanning two starts since his trade from the Cubs.
Don't fret just yet, said Orioles manager Buck Showalter.
"Trust me, we had Ham -- he's good people and a good pitcher," Showalter said. "They made a good acquisition and they'll like him. We were just fortunate what mistakes he made. He's one of those guys you've got to get something early off of."
"I'm not pitching like myself right now," Hammel said. "I need to get back into what I do well, and that's fastball command and harnessing the slider."
The A's, once again, wouldn't go down without a fight, putting two men on base with two outs in the ninth, but there would be no walk-off homer like there was in Friday's thrilling opener.
"We always have that feeling," said Jed Lowrie, who contributed a two-run double in the fourth and also drove in a run in the sixth following Josh Donaldson's double. "That's what makes this team fun to be a part of. Doesn't always work out, but I think if we continue to have that fight, it puts us in the right mindset."
"That's an attribute that this team takes pride in," said A's manager Bob Melvin. "That no matter what happens early on, we're going to try to peck away a little bit and get the tying run at the plate."
Brandon Moss pulled into the team lead for homers with his 22nd in the third, a solo shot to right field off southpaw Wei-Yin Chen that got the A's on the board.
Baltimore plated two runs off Dan Otero in the fourth, courtesy of Jones' two-run base hit through the middle, and one more against a familiar face in Jim Johnson in the ninth.
Johnson served up a loud homer to ex-teammate Davis, his ERA moving to 6.25 since the Orioles packaged him to Oakland in the offseason.