Although the right-hander lost his job as closer last season for the Giants' struggling bullpen, he provides A's manager Bob Melvin with some options. Casilla could be called upon to help handle ninth-inning duties along with regular closer Sean Doolittle, and Ryan Dull could also be in the mix.
"To be able to sign an experienced late-inning reliever who has performed in multiple World Series games makes our bullpen deeper and obviously better," Melvin said by text message. "Great sign for us."
Casilla, 36, has spent his entire big league career with the Bay Area teams; his initial six seasons were with Oakland.
He went 2-5 with a 3.57 ERA and 31 saves last season after posting a 4-2 record with a 2.79 ERA and a career-best 38 saves in 2015.
Left-hander Ross Detwiler and outfielder Alejandro De Aza agreed to minor league contracts with Oakland, with invitations to big league spring training. Outfielder Brett Eibner was designated for assignment to clear roster room for Casilla.
In January 2000, Casilla first came to the A's as a nondrafted free agent from the Dominican Republic. He went by the name of a friend, Jairo Garcia, using his birth certificate to appear younger and thereby more appealing to scouts.
The A's didn't know the hard-throwing pitcher they signed wasn't Garcia -- not until Casilla finally decided to tell the team, through his agent, in 2005.
Later, in a lengthy interview in Spanish with the Associated Press, Casilla shared his regrets over something he considered an act of desperation. At the time, he said, it seemed like the only way to achieve his dream of pitching in the major leagues.
When he returned to the team in 2006 as Santiago Casilla, many wondered about the A's new pitching prospect. But to his teammates, he was always Willie -- his nickname since childhood.
He has long since moved on from that, and now he will begin a new chapter in his career, which already includes three World Series rings -- in 2010, '12 and '14 -- with the Giants.