Today is the deadline for teams to exercise or decline options. The Orioles declined left-hander Tsuyoshi Wada's $5 million option on Saturday.
Casilla receives a $200,000 buyout and becomes a free agent.
The Orioles selected Casilla, 29, off waivers from the Twins on Nov. 2 to provide depth at second base and shortstop. He appeared in 62 games, batting .214/.268/.295 with four doubles, one triple, one home run, 10 RBIs and nine stolen bases in 11 attempts.
Casilla's season ended on Sept. 23 at Tropicana Field after he slammed his head into Nick Markakis' leg while chasing a fly ball in shallow right field. He also jammed his thumb on the play.
Casilla underwent a CT scan in St. Petersburg, Fla. after exhibiting concussion symptoms, and manager Buck Showalter later expressed regret at allowing the veteran infielder to stay in the game. Casilla told reporters the next day that he didn't remember the collision or returning to the clubhouse.
The Orioles began today with 38 players on the 40-man roster. Casilla has been removed, and they didn't extend $14.1 million qualifying offers to eligible free agents Jason Hammel, Brian Roberts and Nate McLouth. Those players also come off the 40-man, along with free agents Scott Feldman, Michael Morse, Francisco Rodriguez and Chris Snyder.
Keep in mind that the Orioles can re-sign any of them at a later date.
In his first full major league season, Machado batted .283 with 51 doubles, 14 homers and 71 RBIs in 156 games. He isn't expected to be ready for opening day 2014 after undergoing a procedure on Oct. 14 that included an arthroscopy and open reconstruction of the medial patellar femoral ligament in his left knee.
Reimold batted .195/.250/.336 with three doubles, five homers and 12 RBIs in 40 games. He spent almost two months on the disabled list with a torn hamstring and underwent corrective surgery on July 23 to fuse two vertebrae in his neck.
Reimold is arbitration-eligible after being paid $1 million in 2013, and the Orioles must decide whether to tender him a contract.
Update: The Orioles made the Casilla decision official a few minutes ago.
Also, MASNsports.com's Steve Melewski received confirmation that left-hander Chris Jones will be added to the 40-man roster.