 After his loan spell finished at the end of the 2006 season, he returned to Bohemians but had failed to appear for the first team during the 2007 season only making under-21 appearances.In need to restart his flagging career, O'Brien departed Bohemians to sign on loan for nearby rivals Shelbourne in July 2007.Mark made his Shelbourne debut on 6 July 2007 against Wexford Youths at Tolka Park in a 1–0 win for Shels and he scored his first Shels goal during a 4–1 victory over Athlone Town at Lissywoollen on 12 October 2007.