Righty Philip Humber, whom the A's claimed off waivers from the Royals on Dec. 17, was designated for assignment to make room for Moscoso on the 40-man roster.
The 27-year-old Moscoso most recently pitched for Caracas in the Venezuelan Winter League, compiling a 3-0 record and 2.66 ERA in 10 games, eight of which were starts. That stint followed a 2010 campaign in which Moscoso went 7-7 with a 5.18 ERA in 23 games -- 22 starts -- for Triple-A Oklahoma City. He struck out 107 and walked 49 in 123 1/3 innings.
Moscoso boasts 11 career Major League appearances -- only one of which came last year, following a 2009 season that resulted in a 3.21 ERA in 10 games with the Rangers. That lone '10 appearance lasted just two-thirds of an inning against Baltimore on May 20, when he allowed two runs on two hits and two walks for a 27.00 ERA.
Kelly was recently acquired by the A's from Pittsburgh in a Minor League deal that sent speedy infield prospect Corey Wimberly to the Pirates less than a month ago. He was 2-3 with four saves and a 4.20 ERA in 38 games -- all but one in relief -- with Class A West Virginia in 2010.