There are different types of OCD. A hoarder (i.e. collecting pringles containers because of the obsessive notion that you might need them someday), a counter (things need to be done a certain number of times) a checker (checking and re-checking that doors are locked or the stove is off, a washer (washing your hands several times a day due to an obsessive fear of germs) and much, much more.
A person with OCD doesn't necessarily have all of these. And, vice versa, it is quite possible to have more than one. I, for example, am a checker, a counter, a washer, and more.
I check and re-check my car to make sure it is locked every time I leave it. I lock it, check the door, unlock and re-lock it, check the door, do it all over again, and then, just for good measure, press the lock button one more time before I'm able to leave.
I have to do things in even increments. The volume in my car gets turned up in two's. However, there are exceptions. Whenever I dry my hands in a public restroom, if I have to grab three paper towels. Two isn't enough to dry my hands sufficiently. Four is too many and it gets bulky and, therefore, I can't dry my hands sufficiently. You'll notice that it all hinges on getting my hands completely dry, but that comes into play when we get to compulsively washing my hands.
Speaking of washing my hands, I wash my hands countless times a day. The only reason they're not dry and raw is because I keep them covered in lotion all day long. Which is where drying comes into play. I have to keep my hands dry at all times. I have this obsessive fear that if they're wet, germs will stick to them and I'll die, or I'll spread some killer virus and everyone else will die, and it all could have been prevented by just drying my hands.
And the kicker: the ever elusive "and more." I have recently developed a new habit. I have an obsessive fear that my hands will fall out through my fingertips. Not just any fingertips - my thumb and forefinger. And it's in a specific spot on each hand. Yes. I said it. I'm afraid my HANDS are going to fall out through my FINGERTIPS. Therefore, I keep my thumb and forefinger pressed together all day and all night to "plug the holes." It's been going on so long I've actually got blisters in those spots.
This isn't everything, and it's just a quick overview, which should give you a bit of insight into just how complex of a disorder this really is.
