Initially a part of New South Wales since his debut in 2001-02, Aaron O'Brien later moved to South Australia in search of greener pastures. The move paid off as he had the most successful years of his career with the new state.

O'Brien performed consistently in the domestic first-class competitions, but has been even more prolific in the one-day format. A left-handed batsman, “Obi” became one of the most integral parts of South Australia's middle-order and also developed into a useful left-arm orthodox spin bowler. However, it was in the Twenty20 format that he truly shone. O'Brien is considered to be a dangerous and economical T20 specialist.