O'Malley (shoulder) will start his rehab assignment with Rookie-level Arizona on Saturday, MLB.com's Greg Johns reports.
O'Malley has been out this entire season after undergoing arthroscopic surgery on his shoulder in May. The shortstop resumed baseball activities in early June and finally makes his return to the field this weekend. There has yet to be a clear timetable for O'Malley, but the 29-year-old will likely return to the big-league club around the beginning of August if all goes according to plan.