If the U.S. is highly stratified on the basis of race, how do we explain the success of people such as Oprah Winfrey and Condoleezza Rice?
This is a good question and I can see from where you are coming. You could also mention people like President Barack Obama and many others like Cornell West, a professor of Religion at Princeton University.
With this stated, you need to consider this important perspective. There is a difference between exceptions and general statements. So, to state that America is stratified based on race is not incompatible with exceptions such as the people you mentioned. In other words, just because there are exceptions (black Americans achieving greatness in society), it does not mean that social stratification based on race is not real. It would be a non-sequitur, to reason in this way.
To prove that social / economic stratification does not exist, you would need to look at all the data, based on numbers, that there is equality among the races.
I agree with that. I have personally heard racist remarks towards all the people mentioned above. There is not equality among races though things are better than they used to be. I am inclined to believe that the success comes from perserverence on the part of the achiever and support from non-African Americans.