If this person fears being a cat, and asks you all the time "Am I cat?" then he is seeking reassurance. What you'll want to do, is tell him something like "You know this is your OCD. You know this is irrational. You know you are anxious; that the anxiety is exasperating your OCD. You are obsessing - distract yourself." the more he obsesses, the more prominent it will be; if left to fester, he may even become convinced that he is a cat. It would help me for someone to say something like that to me. Unfortunately, we're not all worried that we may be a "cat", so it's probably not going to be as black 'n white as that. If this person is in CBT, it might be worth speaking to the therapist, and getting advice. I'll actually, if I remember, ask my psychologist, and see what she says.