The main direction of his philosophical work lay in the field of epistemology and the philosophy of mind.
He was more widely known in South Africa for his moral, political and religious essays,[1] and was described by André Brink as a thorn in the flesh of the establishment.
)[1] Through this he is credited, alongside his contemporaries Johan Degenaar and James Oglethorpe, with introducing the work of the Danish philosopher Soren Kierkegaard to Stellenbosch.
From 1951 to 1955 he did research in philosophy at the University of Amsterdam, and in June 1955 he passed cum laude in the doctoral examination with a study on the phenomenology of Husserl.
Oosthuizen wrote prolifically, presenting his work primarily through public addresses, seminars and lectures.
Oosthuizen's writing as an Afrikaner intellectual have been regarded by the philosopher Johan Degenaar as being on a par with the best work of N P van Wyk Louw in this genre.