Sometimes, when a person is calling someone on the telephone, and announces their name, the other persons says, "OK", instead of "hi" or "nice to hear fom you", etc. OK, when used in this way, is always a very rude way of saying that they do not wish to talk with you, even though you had done nothing wrong to them. Terrible and rude.
Nice person calling a friend:
Sam "Hello Howard, this is Sam."
Howard "OK" (victimizing)