As an un-signed free agent over the summer, O'Brien opted to return to familiar settings in accepting a professional try-out contract to start the 2017–18 season with inaugural AHL club the Belleville Senators, affiliate to the Ottawa Senators. After playing in the opening four games of the season, O'Brien's try-out was successful as he secured a one-year AHL deal with Belleville on October 19, 2017. On February 25, 2018, the Ottawa Senators signed O'Brien to a two-year, two-way contract and was then assigned to the Belleville Senators for the remainder of the 2017–18 season. O'Brien was called up to Ottawa on February 26, 2018 after Nick Shore was traded. O'Brien appeared in 10 games, registering 1 assist in his return to the NHL.