1. Short for out of office, OOF is used to let others know someone is away from work and will be unable to handle any work. Below is an example of an out of office reply.
I'm sorry, but I am unable to reply to your e-mail immediately. I am currently out of office, but will be returning 6/24/2013 and will get back to you as soon as possible.
2. OOF is a term used in place of a swearword or other expression that is difficult to express in chat. Below is an example of how this could be used in chat.
OOF! This is hard.