Magnuson made his major league debut May 17 and appeared in nine total games for the A's this past season, posting a 6.14 earned run average. He also went 4-2 with a 2.98 ERA in 30 appearances for Triple-A Sacramento.
The A's designated Magnuson for assignment on Wednesday. They had acquired the right-hander from Toronto after the 2010 season, as part of the Rajai Davis trade.
In other team news, the A's have agreed to terms with pitcher Edgar Gonzalez and infielder Wes Timmons on minor league contracts for the 2012 season. Both will be invited to spring training.