Mark Leo O'Brien (born 20 November 1992) is an Irish former professional footballer who played as a defender. He played in the English Football League for Derby County, Luton Town and Newport County and in the Scottish Premiership for Motherwell.
O'Brien started his career at Cherry Orchard, before he was spotted by Derby County scouts in the Dublin & District Schoolboy League.
After signing for Derby, O'Brien became a regular in the club's under-18 team in 2008–09, before making his professional debut on 3 May 2009.