Pistorius became the first double-amputee to compete in the Olympics and Paralympics when he ran in heat one.
The athlete known as 'Blade Runner' due to his prosthetic legs finished second in 45.44.
The 25-year-old had looked set to miss out on a place in the individual event after failing to run a second 'A' qualifying time to gain automatic selection.
I didn't know whether to cry, I had a mixture of emotions. It was the most amazing experience.Oscar Pistorius
He fell agonisingly short at his final attempt in July, but was selected for his country's men's 4 x 400m Relay team - in which he won a silver medal at the World Championships last year - which meant he could be selected for the individual event.
Pistorius said: 'I was so nervous this morning. Thanks to everyone for showing their support. I didn't know whether to cry, I had a mixture of emotions. It was the most amazing experience.'