Before entering parliament, O'Brien had held senior positions for Elders IXL, Royal Automobile Association of South Australia and University of Adelaide as well as having been a small business owner. He has Bachelor of Arts and Master of Business Administration degrees and is parliamentary secretary to the Premier of South Australia.
In mid 2007 O'Brien was appointed Chair of the State Planning Review. O' Brien is also Convenor of the Premier's Food Council and Co-Chair of the SA Business Development Council.
The 2006 election saw O'Brien gain a swing of 10.0% increasing his margin to 24.3%.