I've wondered about that.
As I understand it, the following things that I do would be somewhat OCD:
1) When leaving for a while, I check to make sure that the refrigerator and freezer doors are closed, that the stove is turned off, and that the water in the crapper isn't running. If it is cold out and I had an electric heater on, I make sure it is turned off. If leaving from the office, I make sure that the computer monitors and the printers are turned off.
2) I double or triple check the door to make sure it locked, typically returning after I already got in the car and started driving.
3) I often count the number of ceiling tiles, the number of lights, and the number of windows in a public place. Sometimes I count the number of chairs as well.
4) At church, the numbers of each hymn we will sing in the service are posted on a board near the pulpit. I factor each hymn number and then I add the numbers up for the service and then factor that. While I'm at it, I calculate the average hymn number for the service as well.
5) At church, in addition to the number of lights and the number of windows (no ceiling tiles), I count the number of floor tiles from one wall to the other and then the number of floor tiles per pew and the number of pews and use this figures to estimate the width and length of the main part of the church. I usually sit at the very back against the wall and from there I count the number of people at church.