Mark O'Brien is a young defender who captained Derby County's U18 team during 2010-11 as well as appearing regularly in the Rams' reserves. Mark made his Football League debut as a 16-year-old in May 2009. He was the club's Scholar of the Year in 2008-09 but medical problems saw him miss a large part of the following season. Mark was back in action early in 2010 and was included on the first-team bench in the early weeks of 2010-11. Mark added to his first-team experience with a couple of brief appearances from the bench as he continued his development during the season. Mark has represented the Republic of Ireland at U16 and U17 levels.