The short answer is that, starting in 2017, members of a discussion board called 4chan mounted one in a series of attempts to “troll” — or mock — liberals with a right-wing hoax, claiming that the OK sign was a symbol of white supremacy, according to Mark Pitcavage, a senior research fellow at the Anti-Defamation League. The idea was that “the libs” would take up the cry that the OK symbol was racist and, in so doing, make themselves look ridiculous.
Some news outlets took the bait, and the claim was repeated on social media, Pitcavage said. Soon more right-wing trolls joined the campaign, which was anti-liberal but not necessarily white supremacist. Then some actual white supremacists took up the OK sign, apparently still largely as a way to make fun of liberals. And finally, some white supremacists started using the sign without apparent irony, most famously the New Zealand mosque shooter, a self-described racist who killed 51 Muslim worshipers March 15.