If I'm reading the press release correctly, Oprah will do an Iowa rally on Dec. 8 and then hit both New Hampshire and South Carolina on Dec. 9.
I wouldn't claim that Oprah can win an election for anyone -- and it's unclear if she'd help at all in a state like New Hampshire -- but she does a couple of things for Obama. Firstly, she's the sort of endorser that the other candidates -- yes, even the notorious Clinton machine -- wouldn't think of knocking. You don't mess with the Oprah. Secondly, she could give Obama a healthy boost with the black vote in South Carolina. Up to 50% of those voting in the Democratic primary in South Carolina are black. While that would seem like an instant win for Obama, the Clintons have a great deal of goodwill built up with African-American Democrats and there have been whispers from the start that Obama isn't "black enough."
It'll be interesting to see what Oprah says at these rallies, though it's a safe bet she won't be handing out free refrigerators or cars.