"We're calling this our wildest dream season, because this year on the Oprah show, no dream is too wild, no surprise too impossible to pull off," Winfrey said on the show that aired Monday.
Winfrey said the audience members were chosen because their friends or family had written about their need for a new car. One woman's young son said she drove a car that "looks like she got into a gunfight"; another couple had almost 400,000 miles on their two vehicles.
Making sure the audience was kept in suspense, Winfrey opened the show by calling 11 people onto the stage. She gave each of them a car - a Pontiac G6.
She then had gift boxes distributed to the rest of the audience and said one of the boxes contained keys to a 12th car. But when everyone opened the boxes, each had a set of keys.
"Everybody gets a car! Everybody gets a car! Everybody gets a car!" Winfrey yelled as she jumped up and down on the stage.
The audience screamed, cried and hugged each other - then followed Winfrey out to the parking lot of her Harpo Studios to see their Pontiacs, all decorated with giant red bows.
The cars, which retail for $28,000, were donated by Pontiac.
"A little idea grew into a big idea," Mary Henige of Pontiac told The Associated Press.
She added that Pontiac will pay for the taxes and the customizing of the cars.
In other segments on the show, taped Thursday, Winfrey surprised a 20-year-old girl who had spent years in foster care and homeless shelters with a four-year college scholarship, a makeover and $10,000 in clothes. And a family with eight foster children who were going to be kicked out of their house were given $130,000 to buy and repair the home.
"The Oprah Winfrey Show," which debuted in 1986, is syndicated to 212 domestic markets and 109 countries and is distributed by King World, which along with CBSNews.com, is a unit of Viacom, Inc.