Meaning of “oh” - Learner’s Dictionary

oh

exclamation us uk /əʊ/
"Do you go out much?" "Oh, not very often.""Remember to unplug the cable." "Oh yes, I almost forgot.""I forgot to bring your book." "Oh never mind.""What time is it?" "Oh, I think it's about two o'clock.""Do you think Sean will be there?" "Oh, I doubt it."
PAUSE

A1 used before you say something, often before replying to what someone has said:

"Ian's going." "Oh, I didn't realize."
"I'm so sorry." "Oh, don't worry."
EMOTION

A1 used to show an emotion or to emphasize your opinion about something:

Oh, no! I don't believe it!
"I don't think I can come." "Oh, that's a shame."
Oh, how sweet of you!

(Definition of “oh” from the Cambridge Learner’s Dictionary © Cambridge University Press)