What is a singleton pattern? When/Why would you use it? What are the positives/negatives in using that pattern?
What is a strategy pattern? When/Why... (etc)
What is a factory pattern? When/Why... (etc)
Pick a couple of the more "common" patterns and see if they have at least been exposed to them. You can also give a small, bad programming example and have the candidate refactor it. You can see how they approach a problem.
Ask them to draw a picture of one of the patterns on a white board using UML (doesn't have to be perfect, but will show if they have actually used any of it).
When I interview developers I would ask them about version control systems. I didn't really care what they used, but that they actually did use them (and their attitudes towards them). I know that's not OOD, but I just threw my 2c in :)