A former South African Air Force captain, Oosthuizen was elected to the apartheid-era House of Assembly as a member of the National Party in 1987.

[2] He attended the Rand Afrikaans University, completing a Bachelor of Arts in political science and an Honours degree in development studies.

[3] Oosthuizen left the NP caucus at the June 1999 general election, and in October 2000 he became a member of the governing party, the African National Congress (ANC).

[2] Just over three months later, on 16 February 2001, he returned to the National Assembly, now as an ANC representative; he filled the casual vacancy that was created by Fihli Mbongo's death.

He was sworn in late – on his birthday, 10 May – because President Thabo Mbeki had initially offered the post to Vincent Ngema of the opposition Inkatha Freedom Party.