http://msexchangeteam.com/archive/2004/07/12/180899.aspx
I came across this post this morning...
You've heard the term "Out of Office" or OOF for short. Why isn't it OOO for Out Of Office? Read the blog post for the full story....
"OOF was a command used in the days of Microsoft’s Xenix mail system, which set a user as ‘Out of Facility’ - ie Out of the Office."