The switch-hitting Casilla, 28, is a career .250/.305/.334 hitter in parts of seven seasons with the Twins. He's amassed 72 doubles, 14 triples, 11 home runs and 147 RBIs in 1,580 at-bats, and he's stolen 71 bases in 80 attempts.
Casilla has played 399 games at second base, 77 at shortstop and 10 at third base.
The Orioles are hoping that Brian Roberts stays healthy in the final year of his contract, but they can't count on it after he's been limited to 115 games the past three seasons, including 17 in 2012. He's recovering from hip surgery.
Casilla appeared in 106 games this year, batting .241/.282/.321 and going 21-for-22 in stolen base attempts. He also had a 5.33 range factor per nine innings at second base.
"We like his speed and defense, and also, he's a very accomplished base stealer," said executive vice president Dan Duquette. "We think his skills will be additive to our team."
Duquette appears to be done searching for second basemen, which would remove the Orioles from consideration for free agent Jeff Keppinger, who's been linked to them.
"I think we have enough people on our roster to man the position," Duquette said.
Roberts, Casilla and Ryan Flaherty are candidates to start at second base. The Orioles must decide whether to tender Robert Andino and Omar Quintanilla contracts.
The Orioles now have 37 players on their 40-man roster.
Update: The Orioles outrighted outfielder Lew Ford, infielder Steve Tolleson and left-hander Zach Phillips to Triple-Norfolk, removing them from the 40-man roster. They also reinstated Roberts, pitchers Oliver Drake, Stu Pomeranz and Tsuyoshi Wada, and outfielder Nolan Reimold from the 60-day disabled list.
The flurry of moves now leaves the Orioles with 39 players on their 40-man roster.
Ford, 35, batted .183/.256/.352 with three home runs in 25 games for the Orioles in 2012, his first major league action since 2007 with Minnesota.
Tolleson, 29, batted .183/.227/.310 in 29 games over four stints with the Orioles in 2012.
Phillips, 26, pitched to a 6.00 ERA in six games and six innings with the Orioles in 2012. He made 42 relief appearances for Norfolk, going 2-2 with a 3.17 ERA.
Drake, 25, was recalled on Sept. 29 and placed on the 60-day DL with right shoulder tendinitis. He made three starts for Double-A Bowie, going 1-1 with a 1.50 ERA in 18 innings.
Pomeranz, 27, pitched to a 3.00 ERA in three appearances and six innings for the Orioles. He was placed on the DL on May 26 with a left oblique injury.
Wada, 31, had ligament-reconstructive surgery on his left elbow on May 11 and missed the entire 2012 season.
Reimold, 29, batted .313/.333/.627 in 16 games for the Orioles before being placed on the DL on May 1 with a herniated disc in his neck.