Frequency: The word is one of the 1500 most common words in English.
1.
You use ~ to introduce a response or a comment on something that has just been said. (SPOKEN)
‘Had you seen the car before?’—‘Oh yes, it was always in the drive.’...
‘You don’t understand!’—‘Oh, I think I do, Grace.’
CONVENTION
2.
You use ~ to express a feeling such as surprise, pain, annoyance, or happiness. (SPOKEN)
‘Oh!’ Kenny blinked. ‘Has everyone gone?’...
Oh, I’m so glad you’re here.
EXCLAM feelings
3.
You use ~ when you are hesitating while speaking, for example because you are trying to estimate something, or because you are searching for the right word. (SPOKEN)
I’ve been here, ~, since the end of June...
CONVENTION