Casilla, who the A's originally signed as an amateur free agent in 2000, pitched his first six Major League seasons in Oakland before jumping across the Bay Area to San Francisco in 2010. The 36-year-old spent the last seven seasons with the Giants, winning three World Series and serving as the closer for the 2014 championship team.
Casilla had a 3.57 ERA and 31 saves for the Giants last season, but he was part of a rocky San Francisco bullpen that blew a Major League-high 30 saves in the regular season and two more in their National League Division Series loss to the Cubs.