It's time I got back to the interviewing tips as well. So here's an OOP question a suprising number of candidates miss:

Question: In C#, what's the virtual keywork used for.

Answer: A method marked virtual has an implementation, but inheriting objects may override the implementation and provide their own implementation. This differs from the abstract keyword in that the base class does not have an implementation, and the deriving class must provide an implementation.

This is a key concept to object-oriented programming. If you missed this one, you can get more information on Microsoft's MSDN site.