Both Winfrey and former U.S. President Bill Clinton attended a dinner in Hyde Park, catered by celebrity chef Gordon Ramsay, in honor of former South African President Nelson Mandela's 90th birthday.
"There was a very cold reception between them," a source told the Daily News.
"Oprah and Bill, who used to be very close, barely acknowledged one another" when Winfrey approached Mandela's table, where Clinton was sitting, the insider said.
However, other sources in the Clinton and Winfrey camps told the newspaper the pair spoke to each other at a private reception before the dinner.
Robert De Niro, Elton John, Neil Diamond, Kim Cattrall, Pierce Brosnan and Uma Thurman also attended the event.