Johnson, 31, posted a 7.14 ERA with 28 strikeouts and 23 walks over 40 1/3 innings this season. He was designated for assignment earlier this month.
Oakland must pay Johnson, who is free to sign with any team, his $10 million salary.
Johnson was named to the American League All-Star team in 2012 and recorded 101 saves over 2012-13 with Baltimore. He was traded to Oakland in December.
- Chris Johnson