She has such a massive influence on the people who watch her show that you could ask anyone who's ever worked at a bookstore, and they would probably have a story about a customer who wanted to purchase a book, but had no idea what the book looked like, or what it was even about. All the customer knows is that "Oprah said it was an amazing book" so they had to have it. Heck, the beef industry sued her for defamation simply because she said she was going to stop eating beef due to Mad Cow Disease fears (which would most likely cause hundreds of thousands of people to stop eating beef as well, just because Oprah was going to stop).
The famous car giveaway came with a pretty hefty price tag for the winners. When reporting the cars on the show's tax forms, they were listed as "prizes" instead of "gifts", and being prizes, the recipients became responsible for an additional $28,500 on their personal income, which led to a lot of audience members either forfeiting the cars entirely or selling them off to cover the income tax.
HSQ: On episodes where she gives a ridiculous amount of stuff away, like her "Favorite Things" episodes and the episode where everybody in the audience got a free trip to Australia. The last few episodes of that nature had paramedics on standby.
Let's not forget the episode where she gave everyone in the audience a brand new car.