 At the end of the 2017–18 season, his contract was extended by Millwall after the club exercised an option.On 6 February 2013 O'Brien scored on his debut for the Republic of Ireland U21s against Netherlands in a 3–0 win at Tallaght Stadium.O'Brien then scored his first competitive goal and his third goal for Ireland in 4–1 away win against the Faroe Islands.