Shriver's support comes after her uncle, Sen. Edward Kennedy, issued his own endorsement of Obama, and her husband, Republican Gov. Arnold Schwarzenegger, backed GOP candidate John McCain.
Shriver told an audience of 9,000 at the University of California, Los Angeles, that she decided at the last minute to come to the rally, which also featured Obama's wife, Michelle, and Caroline Kennedy, daughter of former president John F. Kennedy.
Other Kennedy family members are supporting Clinton.
The crowd inside UCLA's Pauley Pavilion came to see Winfrey, who offered women a permission slip to desert the historic candidacy of Hillary Rodham Clinton, telling them not to feel guilty if they preferred him over her.
"Being free means you get to think for yourself," she said, "and you get to decide for yourself what to do."
Obama has come within striking distance of Hillary Clinton in California, the biggest delegate prize of Super Tuesday, but 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.
Winfrey bridled at criticism she received after her first campaign foray for Obama in three early voting states.
"You know, after Iowa, there were some women who had the nerve to say to me, 'How could you? How could you?"' she said, with mock indignation. "'You're a traitor to your gender."'
The crowd booed.
"Yes, that's how I feel," she said, adding a little later, "I say, I am not a traitor. No, I'm not a traitor. I'm just following my own truth, and that truth has led me to Barack Obama."
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 was 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.
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."