The 31-year-old O'Flaherty, who pitched for the Braves from 2009-13, was on his way to join the team on Sunday.
Atlanta general manager John Coppolella says ''We've been trying to find a left-hander all spring. 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.''
O'Flaherty, who had Tommy John surgery in 2013, had signed a minor league deal with Pittsburgh last month. He has a 3.24 career ERA in 435 appearances with four teams.