It started as a 4chan prank to trick liberals, but ended up being taken just as seriously by Nazis, KKK, and white supremacists. It was called Operation O-KKK and the plan was to flood Twitter using, and I quote, "basic white girl names" accounts and tweet that the OK sign is a white supremacy symbol.
Later, the OK sign became like a signal to these white power people that you were a sympathizer or member. I don't know how true this is, but at this point the OK sign were said to represent the letter W and P, for white power.
During this time I remember a lot more memes used by these losers. I think Pepe? I don't remember much else.