You say you are OK when being asked if you are OK, which might mean that you are not really OK.

a keeper of a pub

When you hold on at the very limit after much endurance, the moment someone says to you, "Are you OK? Can you keep on going?",  you collapse. On the contrary, when you are really all right and asked if you are all right, you only get confused. That's why you should say you are OK even if you are not really OK to hold barely yourself together.   

November 22,  2016

from “Oriori no Kotoba” by Kiyokazu Washida, The Asahi Shimbun