Dempster, born in Kettering, was signed from Rushden in January 2006 as part of the deal that also brought Andy Burgess to United. He made his debut against his former club and endeared himself to Oxford fans by getting sent off for an assault on his former team mate Darren Caskey. It was only when Jim Smith became manager that Dempster began to consolidate a first team place, but he was unable to hold it down the following season. In January 2007, Dempster went on loan to Conference North side Kettering Town, and the following month his contract with Oxford was terminated `by mutual consent`. Scotland Youth international; Scotland Under-21 international.