Horton, 27, is hitting .224/.246/.373 this season in 16 games at Triple-A Sacramento. He's a career .274/.349/.369 hitter in seven minor league seasons.
The A's chose Horton in the second round of the 2007 First-Year Player Draft out of the University of North Carolina.
Horton has played 386 games at shortstop, 67 at third base and 66 at second base.
The Orioles will assign Horton to Double-A Bowie, which recently lost infielders Buck Britton and Zelous Wheeler to promotions to Triple-A Norfolk.
The trade was first reported by Susan Slusser of the San Francisco Chronicle.