Miller was designated for assignment by Oakland on Dec. 23 after the A's received four players for pitcher Gio Gonzalez.
The 26-year-old Miller went 3 for 12 with a homer and two RBI for Oakland last season after being called up in September. He played in 110 games with Triple A Sacramento, battiing .276 with 32 home runs and 83 RBI. Miller also stole 16 bases without being caught.
Orioles president of baseball operations Dan Duquette says Miller will compete for a roster spot this spring.
In another deal Tuesday, Baltimore traded infielder Brandon Snyder to Texas for cash. Snyder played in six games with the Orioles last year.