I have never heard of one medicine being specifically recommended for pure O. All the SSRIs , plus Anafranil, are used for OCD - often it is a trial and error thing.
But I think it's good you're asking the question, because who knows, maybe we'll find that a lot of people with pure O did have luck with one particular med?
Personally I take Prozac and it has done wonders for my OCD.
Kathrin