Casilla is expected to be a set-up man for Oakland, depending on the health of closer Sean Doolittle.
Casilla recorded a career-best 38 saves in 2015 and followed up with 31 last season. But he lost the closer's job late in the season after blowing his ninth opportunity and didn't even enter the infamous loss in which five San Francisco relievers blew a three-run, ninth-inning lead against the Chicago Cubs in Game 4 of the National League Division Series.
The Giants recently signed free agent Mark Melancon to be their new closer.
"I have left that in the past," the right-handed Casilla told reporters. "It's a new year, it's a new year. I have left this in the past."
The 36-year-old Casilla recorded 127 saves during a career that began in 2004 with the Athletics. He pitched with Oakland through the 2009 season.
His career blossomed when he had a 1.95 ERA in 52 appearances for the Giants in 2010. His ERA was under 3.00 in six straight seasons before he went 2-5 with a 3.57 ERA last season.
Now he is happy to return to the other side of the San Francisco Bay.
"There is a big part of my heart with the Athletics because they believed in me in the first place when they signed me and they believed in me again when they re-signed me," Casilla said. "They taught discipline, they taught me how to play the game of baseball. There are so many things I have to thank the A's for getting my start in the major leagues."
Oakland manager Bob Melvin sees a versatile performer when he envisions Casilla's role.
"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.
Oakland designated outfielder Brett Eibner for assignment to clear a spot on the 40-man roster. The Athletics also signed outfielder Alejandro De Aza and left-hander Ross Detwiler to minor league contracts.