Jason won the under-23 category, completing the course in 34:29, while Thys clocked 34:47 to win the veterans (40 years and older) category.
Another U23 rider, Marc Pritzen, finished the ITT in 35:12, placing third overall.
The 19-year-old Oosthuizen said ITTs were specifically able to “measure how strong we are” – that is, him and his father.
“Most of the time-trials we have competed in, such as in the Vets & Junior Tour, we have also finished first and second,” Oosthuizen told In the Bunch today.
“We would often be about 10 to 20 seconds apart and the rest of the guys would be minutes behind us.”
It has now reached a point where people want to know “who won between my dad and me”, Jason said.
“That’s like the first question people ask us. Not who won in the under-23 or in the 40-plus categories, but rather who won between us.
“It’s turned into a good and competitive rivalry. We train together and we’re both trying to improve each other, so the competition between us helps us and builds our bond.”
He said the Gauteng champs ITT was a better way to determine their form and strength than just training together.
“It was a good build-up to see if the work we put in is paying off. We both managed to win our categories quite comfortably.”
The road race, on the other hand, did not offer that same sort of comparison of form due to them being involved in “different races with different circumstances”.
“It was a long day in the saddle, just pure suffering all the way, but we couldn’t quite measure the game between my dad and me.
“But there again, my dad won the vets race with flying colours; he dominated.”
Jason Oosthuizen finished second in the U23 race behind Pritzen. He said he felt a bit fatigued after the ITT and their build-up to the national road champs.
“I will get the fine-tuning done in the next three weeks up till SA champs, but this past weekend was a good recon of what it will hold.”
The Krugersdorp local, who recently signed for the TEG Procycling Continental team, said it was the general consensus when he was growing up that he would end up competing against his father.
“We trained together on a daily basis and to have someone to mark yourself against and see how much progress you make was very helpful.
“I looked up to my dad as someone better, stronger and more mature than me and that helped me grow into who I am today.
“He sort of offered that benchmark of where I have to be if I want to be good enough,” Oosthuizen said, adding that as he grew older their times grew closer which was when the competitiveness really started to kick in.
Both Jason and Thys are set to take part in the national road champs next month, during which Thys will turn 43.
Oosthuizen said his younger sister, Chante, who competes in the under-17 category, is also an aspiring cyclist.