"Okie" was used as a slur for 50-ish years or so. It isn't used much anymore, but folks still remember it being used rudely. As an OK native, we would not call each other Okies unless we're friends and there's an understanding that it's tongue-in-cheek. Nobody seriously calls an Oklahoma dweller an Okie, it kind of outs you. Similar to how nobody from San Francisco calls it "San Fran", nobody from Oregon calls it "Ore-gone", nobody from New York City calls it "New York City", etc.