It's depended on so many things. Actually we say "ohh", not only bad news. We need to focus him/ her expression when someone said "ohh". Like when some told me, Hey Rajkumar,"I saw you at the park with a baygirl." Who is she? At this situation I said "ohh" in a slow voice and ask him,"why you didn't join us, she is my nephew." For more example, Someone told me, Rajkumar,"I saw you uncle in the hospital and Maybe he got serious injured." I said, "Ohh" with shocked and little bit loud voice and also ask him, When it was happens and how?