Mr Oosthuizen was a member of the South African Air Force up to the Rank of Captain from 1980 until 1983 and then became Self employed from 1984 to 1987.
He was elected as Member of Parliament for the Consistuency of Pretoria Central from 1987 until 1994and served as Whip in Parliament from 1989 until 1999 and was elected as Member of the first Democratic Parliament in South Africa from 1994 to 1999.
He became a Deputy Chairperson of the Pretoria Region for the National Party from 1994 until 2000; and joined the African National Congress (ANC) on 24 October 2000 and was deployed to the National Parliament on 16 February 2001. He served as member of the ANC Strategy Committee in Parliament from 16 February 2001 to 14 April 2004; and on the Joint Defence Committee from February 2001 to April 2004.
He served as member of the Portfolio Committee on Defence from February 2001 to 2004; and as a member of the Portfolio Committee on Correctional Services From 2001 to 2004.
He also served on the executives of various cultural, religious and community organisations and he played craven week rugby, earning a bursary for tertiary studies, also played squash, athletics and long jump.