 In his stint with the club, O'Brien played eight list A matches, scoring 79 runs at an average of 13.16 with a highest of 42, and taking one wicket for 72 runs; he also played five T20 matches for Nottinghamshire, scoring 31 runs and taking two wickets.During his time with Nottinghamshire, he also played as a professional for Plumtree Cricket Club in the Nottinghamshire Premier League.Cricket Ireland, the governing body for cricket in Ireland, awarded O'Brien a full-time "category A" contract in January 2010.