O'Brien (ribs) went 0-for-3 with a walk and a strikeout in his first game back with Triple-A New Orleans on Tuesday.
A rib injury put O'Brien on the major-league injured list in May and the Marlins ultimately decided he was not worth waiting around for; they designated him for assignment June 20 to make room for Zac Gallen. The 28-year-old O'Brien cleared waivers and has since been cleared to return to active competition with New Orleans, where he is likely to play out the remainder of the campaign, with an outside shot at a September callup.