So someone I know was diagnosed with obsessive-compulsive disorder. Anything I should be aware of? 
Well it is a mental disorder where people repeatedly check things, are very routine, and have certain thoughts repeatedly. 
Oh I see. Is there different severity levels to it?
Yes the a scale called the Yale-Brown Obsessive Compulsive Scale is use. 
Does it affect a lot of people?
It's hard to tell although it is more likely to occur in identical twins. 
Very interesting. I wonder why that is.
We don't really know. The cause is currently unknown.