I have always used OB, butt since we're talking about it...
The .mil seems to always use 3-letter acronyms, even for two-word phrases, or otherwise put a slash between them.
Like, I always see Kinetic Strike as K/S. Or, a program named Patriot Excaliber is called (and written as) PEX.
I could probably think of more if I didnt just get done with 12 hours of work at 1am :P .