"It felt great," O'Flaherty said before the A's played the Nationals on Saturday night. "This is a big step forward. I'm very encouraged."
A's manager Bob Melvin said the team is looking at a mid-June return for the reliever.
"Originally, we were looking at the All-Star break," Melvin said. "There is a lot to be excited about."
O'Flaherty would add yet another arm to an already-talented bullpen. In 2011 with Atlanta, he became the first relief pitcher to record an ERA under 1.00 with at least 70 appearances in a season.
O'Flaherty, who has been on a two-day-per-week throwing schedule, threw all fastballs Saturday, and will likely mix in some of his other pitches in another session before being sent on a rehab assignment.
"The ball came out of his hand real nice," Melvin said. "I'm sure he was excited about it."