The talented southpaw relinquished his IBO belt a week ago, after defending it seven times, to campaign in the light heavyweight class.
“Most definitely I want to do well in this weight class and ultimately win titles as well,” Oosthuizen said on Monday.
“I’m hard at gym everyday to be in great shape. I just need few fights before I can challenge for a world title.”
The 26-year-old former champion described himself as a “champion without a belt” and said it was a matter of time before he would be crowned a champion again.
He sent a strong warning to the champions of his new weight division, and said he was not just there to make up the number.
“To the guys in the light heavyweight division, I’m here to compete. To the champions in this division, such as Bernard Hopkins and Adonis Stevenson, I’m coming after your titles.”
The hard-hitting warrior believed relinquishing the IBO crown and moving up a division was a smart move and a natural development.
Oosthuizen withdrew from a proposed fight in Canada earlier in the year because of a motorcycle accident. His weight ballooned to a point where he might have struggled to even to make the light heavyweight limit.
He had since undergone a rehabilitation programme and was reunited with long-time trainer Harold Volbrecht, who said Oosthuizen had regained his zest for training and was already in good shape.
Despite his problems and indifferent form in his most recent bouts, Oosthuizen has yet to be beaten, with a record of 22 wins and two draws.
Meanwhile, the WBA has ranked him fourth on its light-heavyweight list before he has contested a fight in the division.
His opponent in Monaco, Ngumbu, aged 32, is a former African champion. He has 31 victories and five losses to his name. He lost his last fight on a split decision to Russia’s Igor Mikhalkin in his quest to defend the WBO inter-continental light heavyweight belt.