His recovery was hindered by an infection following the operation but, after initially expecting to miss the remainder of the season, he returned for Leinster's final three games.
O'Brien made six appearances for the Lions last summer including in the final two Tests against Australia but after missing Ireland's summer tour of Argentina to ensure he was fully fit for the new campaign he is raring to go.
"I feel great," O'Brien told the Irish Independent.
"[Missing the tour] gave the shoulder a bit of time to relax. It was good to recharge the batteries.
"Last week we were called back in. It was a tough week's training. It's now all systems go."