Mine is OOO for Out Of Office. I always misread it in my head like a ghost and it takes me a few seconds to process. It also doesn't translate to speech—you have to say the whole thing.
Interested to see if others have similar acronyms they beef with.
Not an acronym, but I've been in a couple of jobs where people use "offline" to mean "not at this meeting", as in "let's take this tangential discussion offline". They would say that during an in-person meeting to take the conversation to digital (online!) media!