Contribution to the Community Award at Derby County's 2012 awards night. On 3 July 2012, O'Brien visited a specialist to see if he would be given the all clear to take part in Derby County's 2012–13 pre-season training. O'Brien was given clearance to take part in pre-season training by the specialist and after the sale of Jason Shackell, Derby manager Nigel Clough stated his faith in O'Brien's ability at Championship level. He returned to full training after Derby arrived back from their summer break at Moor Farm on 12 July. O'Brien made his return in a 1–0 pre-season victory over Burton Albion at the Pirelli Stadium, replacing captain Richard Keogh in the 77th minute. Despite playing in several pre-season friendlies including a start at Northampton Town; O'Brien was unable to feature at the start of season due to swelling on his knee. Nigel Clough said he would be cautious over O'Brien's return to action, initially with the new under-21 team. O'Brien made his return on 1 October, playing 60 minutes for the under-21s against Crewe Alexandra. He made his return to first-team action 10 months after his last appearance on 27 October as a 72nd-minute substitute in a 3–0 defeat away to Peterborough United. O'Brien made his return to the starting lineup for the first time in 13 months in Derby's 2–0 win over Bristol City on 15 December. His "unbelievable character" during the match was praised by manager Nigel Clough as he played 87 minutes in the game and was a key factor in Derby's first clean sheet in seven games. Clough said that "He was tremendous. He was struggling at half-time, so we said 'just get back out for 10 or 15 minutes.' He was then struggling at the 70-minute mark but we told him 'you're going to have to stay on.' He then succumbed to cramp in the end but he showed unbelievable character to play for so long because 13 months since a start is a long, long time at this level." O'Brien kept his place in the team until he picked up an injury in the warm-up in Derby's game at Sheffield Wednesday on 9 February 2013. O'Brien was removed from the starting lineup and was later ruled out for a month due to knee joint injury. In late March, O'Brien was ruled out for a further six months after undergoing knee surgery. O'Brien, as