Turns out OOF is a quirk of Microsoft culture, dating back to the company's pre-Exchange Xenix email system of the late '80s. "Oof" was the name of Xenix's auto-reply feature and a command to call it up. Decades after Xenix transitioned into Exchange Server in 1993, people still say "oof," which, like all good slang, has a malleable usefulness. It's both a noun and adjective.
I always wondered where that came from...