What does "That's O.P." mean? It's an achievement in a game. The abbreviation "O.P." obviously doesn't mean "Original Poster" in this context. I think it may stand for surprise.
Can somebody give me an accurate explanation?
I would say: OP = over-powered
In Military Realism games, it might be used as 'Enemy is Operational Personnel'.
Essentially, that has the same effect as another usage of 'That's OP', which is likely to be referring to a weapon/skill being 'overpowered'.
You may hear players say O.P.O.P, which is Over Powered Operational Personnel. When you hear that specific term run as fast as you can, because the person to whom they are referring is a serious bad ass.