Jackson, headed to Triple-A Nashville after the deal Wednesday, has pitched for 12 major league teams. That is one short of the record 13 by former reliever Octavio Dotel.
After going 5-6 with a 5.21 ERA for Baltimore and Washington last season, Jackson had spent this season in the Nationals system without pitching in the majors. The 34-year-old right-hander opted out with Washington, and his deal with the A's reportedly has an opt-out of June 20 if he doesn't get back in the majors by then.
Jackson has started 288 of his 377 career major league games since his debut in September 2003 with the Los Angeles Dodgers. He has a 98-120 record with a 4.67 ERA.