I use it for... OBSESSIVE TWILIGHT DISORDER!!!! Wooo-Hooo!!!! I have OTD!!!! And much more O...D!!!!
There are also some that I don't know if they even exist but they are:
OCD -> Obsessive Carlisle Disorder
OECD -> Obsessive Edward Cullen Disorder
OJD -> Odsessive Jasper Disorder
...And so on. There can be a lot of O...D that you can create ;)
Wow. Some people. (Points upward at comments.) Although, the caps probably was a bit much. if you had done it without CAPSLOCK , I bet people would be nicer.