(also less frequent O ) / əʊ; NAmE oʊ/ exclamation
1.
used when you are reacting to sth that has been said, especially if you did not know it before :
'I saw Ben yesterday.' 'Oh yes, how is he?'
'Emma has a new job.' 'Oh, has she?'
2.
used to express surprise, fear, joy, etc. :
Oh, how wonderful!
Oh no, I've broken it!
3.
used to attract sb's attention :
Oh, Sue! Could you help me a moment?
4.
used when you are thinking of what to say next :
I've been in this job for, oh, about six years.
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WORD ORIGIN
mid 16th cent.: variant of O .