To be fine; to be not especially good or bad. A: "How have you been?" B: "Oh, I've been OK." These fries are OK, but they're not the best I've ever had.
See also: OK
(I've) been okay.
Inf. a standard response to a greeting inquiry that asks how one has been. Bill: Well, how have you been, good buddy? John: I've been okay. Sue: How you doing? Jane: Been okay. And you? Sue: The same.