I saw part of the show. I ended up changing the channel because they didn't say anything that hasn't been said before. I don't think the general public is going to change their vote based on what Cameron Diaz, Drew Barrymore, and forgettable actress name has to say.
It wasn't biased from what I saw. The last celebrity to come out was P.Diddy wearing his "Vote or Die" tshirt. He explained his reasons behind urging people to go to the polls.
All Oprah did was provide a forum to urge people to vote.