Definition of oh exclamation from the Oxford Advanced Learner's Dictionary
oh
exclamation (alsoespecially literaryO)
əʊ oʊ
1used when you are reacting to something that has been said, especially if you did not know it before‘I saw Ben yesterday.’ ‘Oh yes, how is he?’Oh dear! What's happened now?You can't come tonight? Oh well, see you next week then.‘Emma has a new job.’ ‘Oh, has she?’2used to express surprise, fear, joy, etc.Oh, how wonderful!Oh no, I've broken it!3used to attract somebody's attentionOh, Sue! Could you help me a moment?4used when you are thinking of what to say nextI've been in this job for, oh, about six years.
əʊ 
oʊ 
1 used when you are reacting to something that has been said, especially if you did not know it before‘I saw Ben yesterday.’ ‘Oh yes, how is he?’Oh dear! What's happened now?You can't come tonight? Oh well, see you next week then.‘Emma has a new job.’ ‘Oh, has she?’