Resop was 1-4 with a save and a 3.91 ERA in 61 relief appearances with Pittsburgh last year. He tossed a career-high 73.2 innings, which tied for seventh among National League relievers. The 30-year-old right-hander allowed a .276 opponents batting average, including .237 against left-handed hitters. Resop has spent parts of seven seasons in the majors with Florida (2005-06), the Los Angeles Angels (2007), Atlanta (2008, 10) and Pittsburgh (2010-12). He has a 9-11 career record with two saves and a 4.51 ERA in 213 relief appearances.
Thornton spent the 2012 season at Single-A Stockton where he was 4-0 with 16 saves and a 4.53 ERA in 48 relief appearances. He allowed a .264 opponents batting average and struck out 70 while walking 19 in 53.2 innings. The 24-year-old right-hander was originally selected by Oakland in the 23rd round of the 2010 First-Year Player Draft.
Mitchell was designated for assignment Wednesday. He spent the entire 2012 season at Triple-A Sacramento where he batted .252 with 15 doubles, 11 triples, six home runs and 38 RBI. Mitchell was originally drafted by the A’s in the 5th round of the 2006 draft and is a .281 career hitter in 753 games over seven seasons in the minors.