O'Flaherty, 31, was fighting for a spot in the Pirates' bullpen this spring training after signing a minor league deal last month. He allowed eight runs (three earned) on 15 hits over 9 1/3 innings in 10 appearances this spring.
O'Flaherty, a 10-year veteran, pitched for the Braves from 2009-13.
O'Flaherty, who had Tommy John surgery in 2013, has a career 22-11 record with a 3.24 ERA over 369 1/3 innings in 435 appearances with the Seattle Mariners, Braves, Oakland Athletics and New York Mets.
"We've been trying to find a left-hander all spring," Braves general manager John Coppolella said Sunday. "He's somebody we know. When our scouts saw him and when our field staff saw him, we felt like he was somebody who could help us out. Obviously, we know this guy's great off the field. He was great on it with us. We're hoping he can fill a need a help us get better."