Atlanta transferred O'Day's (calf) rehab assignment from High-A Rome to Triple-A Gwinnett on Tuesday.
O'Day, who struck out a batter in a perfect inning during his lone appearance for Rome on Friday, will likely pitch in one or two games for Gwinnett before coming off the 15-day injured list as soon as this weekend. The veteran right-hander turned in a 4.15 ERA and 1.34 WHIP in 21.2 innings out of the big-league bullpen this season before going on the 15-day injured list July 13 with a left calf strain.