Longman Dictionary of Contemporary English - oh

~ interjection 1 used to make a slight pause, especially before replying to a question or giving your opinion on something  ("What time are you going into town?" "Oh, I haven't decided yet." | "I don't like the new boss." "Oh, I think she's quite nice.") 2 used to get or keep someone's attention so that you can ask them a question or continue what you are saying  (Oh, Janet, could you get me a paper while you're out? | Milk, cereal, juice - oh, and put lettuce on the list too.) 3 used to express a strong emotion or to emphasize what you think about something  (Oh, aren't those flowers gorgeous! | Oh, how awful! | She got the job? Oh great!) 4 oh, did he?/oh, are you?/oh, was she? etc used to show that you did not previously know what someone has just told you  ("Frances has left her husband, you know." "Oh, has she?")