A myelin sheath covers your entire nervous system, not just what's in the brain. If you had one that was wearing away you'd be feeling all kinds of stuff. I doubt anyone knows what causes OCD. A company called Planetary Formulas makes a natural formula for helping to maintain a strong myelin sheath; don't know if it works. If you're really worried about it, only a neurologist and probably an MRI could tell you the answer, but if all you have is OCD it's probably not that.
Not really sure, but you can treat your OCD. Talk therapy can be extremely effective in dealing with this. Learning about your compulsions and why you act the way you do is a very good way to confront it in my opinion.