Now, my major OCD thing is locks. If I go out of town, I have to be absolutely sure I locked both doors on the house, and shut the garage door and closed the driveway gate. I will turn around two miles from my house to check if I'm not 100-percent sure. Annoying yes. but not a big deal. I know it is stupid, but also know it will mess with my head if I don't go back and check. I just plan it into my travel time.
I believe the lock thing is quite common amongst people with OCD. Maybe its because locks represent security and OCD in general is all about feeling secure and not being able to relax until a certain ritual is completed.
My partner who has (fairly early stage) dementia has developed some habits which verge on OCD. His latest one is an obsession with swirling his beer around in the glass all the time to prevent foam adhering to the sides of the glass. I'm sure I've done that sometimes but he does it over and over again the whole time he is drinking. Its driving me mad! He claims it is 'sensible'. If I try to stop him he does stop for a short time - then it starts up again.