He sprinted to victory on the third stage, finishing the 101.7km race in 2:40:53. His teammate Rickardo Broxham wasn’t far behind, finishing 12th in the bunch.
The junior, who finished second on day two, now leads the points classification by four points.
He and Broxham are 20th and 21st respectively on the general classification and their impressive performances have boosted South Africa to 8th in the team GC.
Oosthuizen started the tour fresh from success in Belgium, where he sprinted to victory in Herselt.