Recruited from West Coburg, O'Brien captained North Melbourne's under-19 side for two years before playing a handful of senior games as a half-forward flanker in the 1960 season. He was dropped to the reserves for the whole of 1961. In the lead-up to the 1962 season, O'Brien was invited to train by both Essendon and Footscray. O'Brien formally requested a clearance to Footscray, seeing that as an opportunity to again play at the senior level. This prompted a re-think by North Melbourne; O'Brien was retained, reinventing himself as a half-back flanker and utility player. He excelled in these roles, justifying North's decision to turn down the transfer. O'Brien became a stalwart in the Kangaroo side for the next eight seasons, and in 1964 was runner-up to Noel Teasdale in the Syd Barker Medal, North Melbourne's best and fairest award.
In 1970, O'Brien joined Victorian Football Association (VFA) team Brunswick, and was appointed the dual role of captain-coach at the end of the 1971 season.[3] He led the side to a Second Division Grand Final appearance in 1973, after which he announced his playing days were over.
O'Brien went on to serve in the North Melbourne match committee as a team selector under premiership coach Ron Barassi.
While playing senior football, O'Brien studied to become a licensed realtor and auctioneer, eventually becoming a director and later partner of a real estate business in Melbourne, Victoria. He is now retired and lives in Gowanbrae, Victoria.
^"N-O". Archived from the original on 10 February 2012. Retrieved 19 September 2007.
^"O'Brien coach of Brunswick", The Age, 25 November 1971
References
Holmesby, Russell; Main, Jim (1998). The Encyclopedia of AFL Footballers: Every AFL/VFL Player Since 1897. Melbourne: Information Australia. ISBN1-86350-243-2.