"If you don't want to be called a white supremacist maybe don't hang out with a group where they're heavily represented and use a symbol that they've heavily coopted. Seems...I dunno.... dumb as fuck?"

This would be a reasonable response if it could be proven that he had any idea that they were Proud Boys/white supremacists. You even started your statement by saying they have "coopted" the "OK" sign, meaning they are using it in a way different from it's normal use.

So in the pictures, unless it's proven Rittenhouse knew they're affiliation with racist groups, it's entirely feasible that he was using the OK sign in it's usual context for a photo while others weren't. It's also possible that he was prompted to make the ok sign for the photo, like "Hey Kyle, give us an OK sign to show people you are getting through the trial fine!" or something similar.