After graduating from the Connacht academy, O'Halloran was signed on a development contract for the 2011–12 season. That season saw Connacht qualify for the Heineken Cup, Europe's premier club competition, for the first time. On 11 November 2011, O'Halloran scored Connacht's first ever try in the competition against Harlequins in the opening round. He also featured for Connacht in the 36–10 defeat to Toulouse in and the 23–19 loss against Gloucester, scoring another try against Gloucester. O'Halloran then played in the 24–3 loss against Toulouse but played in the province's first ever Heineken Cup win when Connacht beat Harlequins 9–8. In the 2011–12 Pro12 O'Halloran scored tries in both games against the Cardiff Blues along with tries Leinster and Glasgow Warriors. In Round 20 of matches, O'Halloran scored a try in a 26–21 victory over Ulster to take his tally of tries to five in the Pro12 and seven in all competitions.
