Companies already know an endorsement from Winfrey is worth more than its weight in gold. Getting a book selected for her book club makes it a guaranteed best-seller. For instance, "The Road" by Cormac McCarthy sold just 156,000 units before being selected for Oprah's Book Club in 2007. Afterward, it sold 1.4 million units. "Oprah's Favorite Things," a list of Oprah-approved gifts that appeared on her talk show — and now in her magazine, O — also generates huge profits for the selected products.
Winfrey's negative opinions have major consequences, too. Back in 1995, during an episode of "The Oprah Winfrey Show" about mad cow disease, Winfrey mentioned that she would stop eating hamburgers after a guest said that feeding processed livestock to cattle had been linked to mad cow disease in Europe. A group of Texas cattle producers claimed her comments caused beef prices to tumble and cost them $12 million. They sued her for libel, but she won.
However, Winfrey endorsed both Barack Obama (who won the presidency) and Hillary Clinton (who lost), so her endorsement doesn't mean automatic success.