The Brief: Oof is an exclamation of discomfort.

DEEP DIVE
Oof is an interjection signaling discomfort. Users may incite the phrase when feeling overwhelmed, awkward, or generally at a loss. Oof can be used at the beginning or end of a sentence, or on its own. It is a ubiquitous term that can be used in reaction to almost anything.
oof looks like jungkook wasn’t supposed to go in with jimin, it was hoseok’s turn…eager much? 👀 pic.twitter.com/CUjO3pi9jB
— ❣︎︎ (@jikookfolders) January 29, 2019