Koos matriculated from Grey High School, Port Elizabeth in 1953. He obtained his BA LLB degree from the University of Stellenbosch in 1960.
He did his articles with Dolley and Wink in Uitenhage. After completing his articles, he was employed as a professional assistant with Brown Braude in Port Elizabeth. In 1969 he became a partner with Van der Walt, Olivier and Alberts.
In 1980 Olivier and Alberts merged with Oosthuizen Hazel and Wilmot Inc. and in 1995 Oosthuizen Hazel and Wilmot Inc. merged with Cornish and Bowes.
In 2002 he became a consultant with Oosthuizen Hazel and Wilmot Inc.
He was elected as a Councillor of the Cape Law Society – during that time he served on the Finance Committee, Disciplinary Committee, Candidate Attorney’s Committee and the Legal Services Committee, he was the Chairperson for the Finance Committee and the Legal Services Committee from October 2002 to date.
He served as representative of the Eastern Cape on the Law Society of South Africa from 2005 to 2014.
He served as President of the Cape Law Society from 2013 to 2014.