Oh
It actually means my heart just got ripped into a million pieces but I won't tell you because you wouldn't care how I feel anyway.
Example:
Elizabeth: I love you.
Isaac: I know that but I'm not ready for it yet
Elizabeth: Oh.
Bystander: That's harsh.
Elizabeth: I love you.
Isaac: I know that but I'm not ready for it yet
Elizabeth: Oh.
Bystander: That's harsh.