Here’s an interesting historical question - when we say Out of Office, why does it sometimes get shortened to ‘OOF’? Shouldn’t it be ‘OOO’?
 
Inside Microsoft, ‘OOF’ means not just the message which says you’re Out of Office, but it has grown to mean the act of being Out of the Office too - so you’ll get people putting sticky notes on their door saying ‘OOF Thurs & Fri’ or even people verbally saying things like, "Oh, Kevin’s OOF on vacation for the rest of the week’. I suppose that sounds better than "Oh, Kevin’s OOO on vacation ..."
 
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. The usage of the term ‘OOF’ just stuck, as did the term ‘Little r’ (e.g. on an email sent to a distribution list, "Who wants to go to the cinema tonight? Little ‘r’ if you’re interested", meaning reply just to me) - as preserved in Outlook with CTRL+R for Reply, and CTRL+SHIFT+R (aka Big R) for Reply All.

- Ewan Dalton