For most celebrities, I'd say no. I think that a celebrity's support for a politician is usually both a blessing (money, fame) and a curse (Hollywood telling America what to do), which cancel each other out.
However, with Oprah Winfrey, I'd say yes, her support will help Barack Obama. Oprah has a large fan base who really listen to her. When she endorses a book, it goes to the top of the bestseller list. When she endorses a candidate -- especially when she spends and raises money for that candidate -- there may be a similar effect.