1. Used alone to mean that you feel quite strongly about something that has just happened or has just been recounted to you. It makes you feel ill at ease or squeamish and you feel as if you must contain yourself from reacting or saying something. You don't know what you'll say or do, or you are afraid of offending someone. English = 'Jesus!', 'Dear me!', 'Good grief!', 'Oh, no!', 'Oh, my god!', 'Good god!', 'Ah, yuk!', and so on.
2. Used alone to express the fact that you have just suffered or are suffering some personal discomfort, if not pain. English = 'Ouch!' etc.