Jason Berken pitched shutout ball in four of his five innings Wednesday afternoon.
But he gave up four runs in the fourth inning vs. Oakland and that was enough for the A's to hand him his 10th loss and win the final game of this series, 6-3.
Oakland, which has won seven of its last ten games, takes the series, two wins to one. The A's are 10-1 in their last 11 games vs. Baltimore.
Oakland took a 4-0 lead with a four-hit fourth inning that included two sac flies and Landon Powell's two-run homer.
The Orioles got back in the game with a three-run sixth. Aubrey Huff hit a two-run homer, his 13th of the year. Luke Scott doubled and later scored on an error.
Oakland added insurance runs in the 8th and 9th innings for the final 6-3 margin.
Winning pitcher Vin Mazzaro, now 4-8, gave up three runs and eight hits over 5 1/3 innings. Andrew Bailey pitched a scoreless ninth for his 17th save.
The O's have lost four of five, 10 of 13 and 17 of their past 23 games. After Thursday's off day, the Orioles host Los Angeles for a four-game series that begins Friday night.