The Athletics won the opener of a three-game series that is laden with playoff implications.
Oakland stayed within three games of Texas for the lead in the American League West and Baltimore missed a chance to move in front in the AL East. The Orioles and New York Yankees are tied for the division lead, three games in front of Tampa Bay.
A two-run homer by Yoenis Cespedes gave Oakland a 2-1 lead in the fourth, but Chris Davis tied it in the top of the fifth with his 26th home run of the season.
Baltimore escaped further damage in the fifth, but could not take advantage of excellent scoring opportunities during the next two innings when they had two runners on with one out and could not score.
The Orioles also put their leadoff man on first in the ninth, but Grant Balfour survived to collect his 18th save. The final out of the game came when pinch-runner Xavier Avery was thrown out trying to steal second.
Tommy Milone (13-10) picked up the win by allowing a run on six hits in 6 1/3.