 Domestically O'Brien began his professional career with Kent in 2004 before joining Northamptonshire at the start of 2007, spending 6 seasons there before joining Leicestershire for 2013.He made his One Day International debut for Ireland in 2006; two years later he played his first Twenty20 International.He was one of the eleven cricketers to play in Ireland's first ever Test match, against Pakistan, in May 2018.