Peter John O'Callaghan was born in Horsham, Victoria in 1931 and was schooled until 14 at Horsham's St Michael's and St John's school.  He left to become a motor mechanic, and at age 18 commenced a correspondence course at Taylor's, before enrolling to study law at the University of Melbourne in 1955.

After graduation, he was articled to Brendan McGuinness & Co and remained there as a solicitor until 1961 when he signed the Victorian Bar Roll. 

Peter read with (Sir) Kevin Anderson, and has a wide general practice including crime, liquor licensing, town planning and environment law, administrative law, and major constitutional cases.  He has appeared in many Royal Commissions (such as the Royal Commission into Tricontinental and the Royal Commission into Liquor), Boards of Inquiry, mediations and arbitrations (such as Esso/BHP v.Gascor Limited and General Vic,) , and latterly appeared as counsel in commercial and trade practices matters. 

He was director of Barristers Chambers Limited from 1979 to 1993, and chairman of the building committee responsible for the construction of Owen Dixon Chambers West.

Peter has been a member and chairman of the AFL Appeals Board for over twenty years.  He is also admitted to practice in New South Wales, Western Australia and Ireland.  He has and continues to act as independent expert in building and commercial disputes.

He is the widower of Jennifer since July 2001, with a family of six sons and eighteen grandchildren.