On 1 January 2011, O'Brien signed a permanent deal with Leeds United on a -year contract, which took effect after his loan deal came to an end on 4 January 2011. He turned down a lucrative offer from Cardiff City, opting to move permanently to Leeds. On 8 January, O'Brien put in an impressive performance to help earn Leeds a 1–1 draw against Arsenal, Leeds were 1–0 up when Robert Snodgrass scored a second half penalty, Arsenal equalised in the 90th minute when Cesc Fàbregas scored a penalty. O'Brien picked up an injury in the FA Cup replay against Arsenal and subsequently missed the game against his former club Portsmouth. O'Brien scored his second goal for Leeds in the 3–2 loss against Millwall.