Now, I don't actually know Oprah Winfrey personally, and never watch her show. But I gather that switching how she talks is part of who she is. Consider this chunk from a blog post giving her far more grief than Randi Rhodes on a wide range of topics:
Irritating quality number 617: Oprah's constant dialect shift. Never content to accept her crown as the voice of the goods-crazed white mother, Oprah is known to shift in between several different dialects, accents, and speech patterns during a single broadcast. A somber, intellectual tone is replaced two seconds later with a southern drawl in the midst of Paula Deen. The white Oprah will turn on a dime to morph into black Oprah without the aid of a phone booth for changing purposes.This writer claims to do closed captioning for Oprah's show, and if that's so, has paid close attention to Oprah's speech. I take that to suggest that Oprah's repertoire does include this kind of style.
Rhodes has gone into great detail comparing this to Hillary Clinton's "pandering" with a switch to African-American tinged speech. Leaving aside the wrinkles in that story (long story short: Clinton was mostly reciting lyrics). Clinton surely doesn't speak the variety she was referencing and it's at least plausible that Oprah speaks the one she was using natively or near-natively.