The A's also designated right-hander Kirk Saarloos for assignment. Davis, who was expected to be back in the Bay Area late Wednesday after flying in from his home in Connecticut, was designated for assignment by the Giants on April 20. He could play Thursday in the finale of a three-game series against the Minnesota Twins.
"He plays left, center and right," A's manager Bob Geren said. "He's a good runner. Having a speed guy is something that's a nice addition."
Geren said the decision to bring in Davis wasn't affected by the fact outfielder Travis Buck has been bothered by shin splints and could need a stint on the disabled list. Buck was a late scratch before Tuesday's game with Minnesota and wasn't in the starting lineup Wednesday night, his fourth straight contest sitting out.
The 27-year-old Davis, acquired by the Giants from Pittsburgh for pitcher Matt Morris at last year's trading deadline, made San Francisco's opening day roster but batted only .056 with four stolen bases in 12 games.
A right-handed hitter, Davis broke into the majors in 2006 with the Pirates and has a .252 career average with one home run, nine RBIs and 27 stolen bases in 107 games.
The A's brought Saarloos back for a second stint with the club this season, signing him in January to a minor league contract. He was traded to Cincinnati from Oakland in January 2007 after three years with the A's. He went 1-5 with a 7.17 ERA in 34 games and three starts during four stints with the Reds.
Oakland called him up to the majors on April 14 and he pitched in two games with a 2.35 ERA. He went 2-0 with a 2.45 ERA in two starts for Triple-A Sacramento.
"I would like to keep him when that process goes through," Geren said. "He's done nice work for us."
Meanwhile, Oakland right-hander Rich Harden threw off the mound for the first time since going on the disabled list April 3 with a strained throwing shoulder. He is slated to pitch in a simulated game Saturday in Seattle as his next step.