"I have always represented South Africa with pride so didn't make my decision without a great deal of thought," 2010 British Open winner Oosthuizen said in a statement released by his management company.
"I would like to wish our golfers and all other athletes competing in Brazil all the very best for success."
According to his management company, world number 12 Oosthuizen informed South Africa's Olympic committee of his decision "after long deliberations citing family and schedule issues".
The decisions by Oosthuizen, Scott and Singh were described as "regrettable" by the International Golf Federation (IGF).
"The IGF understands the challenges players face in terms of scheduling this summer and it is regrettable to see a few leading players withdraw from this year's Games," IGF president Peter Dawson said in a statement.
"The Olympics is the world's greatest celebration of sport and it is exciting and appropriate that golf features in its programme again."
Singh, a three-times major winner, announced last week that the Zika virus was his main reason for skipping the Rio Games.
"I would like to play the Olympics, but the Zika virus, you know..." Singh, 53, told Golf Channel. "I feel bad, I wanted to play and finally decided against it. It's in the middle of the Tour over here and I'm trying to figure out my game."
U.S. health officials have concluded that Zika infections in in pregnant women can cause microcephaly, a birth defect marked by small head size that can lead to severe developmental problems in babies.
The World Health Organization has said there is strong scientific consensus that Zika can also cause Guillain-Barre, a rare neurological syndrome that causes temporary paralysis in adults.
The connection between Zika and microcephaly first came to light last autumn in Brazil, which has now confirmed more than 1,100 cases of microcephaly that it considers to be related to Zika infections in the mothers.
For Scott, a hectic playing schedule from mid-June until mid-August was his biggest cause for concern with the last three golf majors this year crammed into a seven-week span because of the Rio Games.
"It's nothing I've ever aspired to do and I don't think I ever will," world number seven Scott told Reuters last year. "It's all about the four majors and I think that's the way it should stay for golf."
The Rio Olympics takes place from Aug. 5 to Aug. 21, with the men's golf tournament ending Aug. 14.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Larry Fine)
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