Oakland announced the moves before Friday's game against the Kansas City Royals.
The 37-year-old Okajima was 0-1 with a 2.16 ERA in 14 games with the River Cats. He last appeared in the major leagues with the Boston Red Sox in 2011.
"I'm very excited about pitching here," Okajima said. "It feels good coming back from Japan. I want to be able to pitch like I did my first year in Boston. I was hoping to get back to the majors and the A's gave me the opportunity. I polished up some of my pitches in Sacramento, like the change and I worked on my cutter, too. There was a lot of traveling to and from Sacramento."
Information from The Associated Press was used in this report.