Oh! Sundo
It is an expression that represents positive astonishment in the following situation: When someone mentions a future event that is too good to be true and people do not expect it to actually happen, but this person unexpectedly and casually returns with the realized result.
Person A:"I think I am going to start a brewery"
Person B:"Yeah why not."
Person A and B meet again a few months later
Person A:"Oh yeah by the way, I started my own organic brewery business. We're going to start selling our products at Whole Foods."
Person B: "Oh! Sundo!"