Arrieta (2-0) made his second major league start Tuesday and held the Giants to one run on three hits over seven-plus innings.
Adam Jones homered for the Orioles, who are 7-26 on the road this season. Miguel Tejada had three hits and scored a run.
Pablo Sandoval accounted for the only Giants run, a solo home run off Arrieta in the third inning. The Giants managed only three hits in the game.
Joe Martinez (0-1) took the loss in his first start of the season. Martinez was touched for four runs on eight hits in 6 1/3 innings.