World number seven Oosthuizen will be the highest-ranked player at the 2.2 million euros (£1.9m) European Tour event, which is co-sanctioned by the Asian Tour.
The two Americans pulled out of the event against a backdrop of political tension as North Korea threatened nuclear attacks on the United States, South Korea and Japan after new UN sanctions were imposed in response to its latest nuclear test in February.
Sponsors of the European Tour event said in a statement on Monday: "They have reached this decision following perceived unrest on the Korean peninsula".
While the situation has eased in recent days, the South's Yonhap News agency reported on Sunday that the North had moved short-range missile launchers to its east coast, indicating it was pushing ahead with preparations for a test launch.
"It was a personal decision the two of them made not coming here and ... you can't really give them any hard time about it," Oosthuizen told a news conference, adding "... both of them made the decision with their families not to come.
Oosthuizen, who flew from his home in Florida to South Korea on a private jet provided by organisers, said he had consulted his management before deciding to come.
"I took the word of the guys that, everything is fine over here and ... I was looking forward to the week and I think something drastically needed to go wrong for me, for me not to come, and, yes, I'm glad I came," he added.
Oosthuizen missed the cut at the Masters earlier this month and said he had to tighten things up around the greens.
"My short game really let me down at the Masters as my putting wasn't good enough. I had far too many putts, and ended up missing the cut by two," he said.
"I hit it okay. It was not great, but not terrible either. The greens just got the better of me for the two days. But it hasn't knocked my confidence too much, and I still feel good about this week."
Former US PGA Championship winner Yang Yong-eun of South Korea, 1999 British Open winner Paul Lawrie and former European Ryder Cup player Paul Casey will also be chasing victory in the event, which kicks off on Thursday.