 Mullins was recalled by parent club Rotherham on 21 November, after they exercised the 24-hour recall option as part of his loan deal, having made eight appearances, scoring twice.On 22 July 2013, Mullins signed for Oxford permanently on a three-year contract for an undisclosed fee.He scored an 82nd-minute winner in a 2–1 win at home to Bury on 10 August, and went on to score a 96th-minute equaliser at home to Wycombe Wanderers two weeks later, followed by a 57th-minute equaliser away to Cheltenham Town on 14 September.