The Illinois Democrat has come within striking distance of Hillary Rodham Clinton in California, the biggest delegate prize of Super Tuesday. But Obama still lags among women.
A Field Poll out Sunday had the race a tossup, with Clinton at 36 percent and Obama at 34 percent. But there was a marked gender gap, with women favoring Clinton by 13 points and men favoring Obama by the same margin.
Since about 55 percent of Democratic primary voters are expected to be women, Obama's campaign is hoping that Winfrey, with her vast, largely female audience, can help bring more of them his way.
Walking into the sports pavilion, 29-year-old Iroro Edos said she prefers Obama but could live with Clinton as president.
"I'm for Democrats. It doesn't matter if Obama or Hillary wins," said Edos, who lives east of Los Angeles in Pomona. "As long as the Democrats win, I'm good."
It will be Winfrey's first appearance on the Obama campaign trail since she held rallies for him at the end of last year in Iowa, New Hampshire and South Carolina.
She was to appear Sunday with Caroline Kennedy, the daughter of former President John F. Kennedy, and Obama's wife, Michelle. Thousands waited in the morning rain to get into the event on the University of California, Los Angeles, campus.
The crowd was black and white, male and female, young and old.
Asked why he came, 51-year-old David Sobodos, of Redondo Beach, answered, "Obama - and I like Oprah, of course."
Sobodos, who used to manage a motor sports raceway in Los Angeles, said he liked Obama's open-mindedness and Winfrey's good works.
"Whatever she connects herself with is about being good," he said. "Helping people, lifting people up."