With OCD and other compulsion disorders, as well as mental disorders such as schizophrenia or MPD, there are key behaviors that occur in every case and that define the "type" of the disorder they have. In schizophrenia alone, there are three main kinds - and you can be a blend of all three or just one.
There are also antisocial disorders, which generally have the same key symptoms exhibited in behavior (sociopaths, agoraphobics, ect.).
Many diseases have physical ailments that pair with them, which can make your writing more in depth, such as seizures, light sensitivity, noise sensitivity, ect.
PTSD (post traumatic stress disorder) is a good one to use if you don't want to follow an actual outline of what a mental character may encounter or experience from their madness, or just a generalized "sociopath," who has no fucks to give over the rules and regulations that define society and also feels no empathy or compassion, even for themselves.
I have played a sociopathic character as well as one who suffered from multiple personality disorder, and I will go ahead and say that I felt much more comfortable writing both of them after reading background information about their disorders than I did previously; it made my writing feel more real and gave me a lot of interesting, muse inspiring information to go off of.