oh · exclamation /oʊ/Full view
oh (NEW IDEA)
A1 introduces an idea that you have just thought of, or something that you have just remembered
Dictionary examples:

Oh, I just realized there might be a problem.

Oh, and don't forget to lock the back door.

Oh yeah, someone called and left a message for you while you were out.

Learner example:

Oh, and don't forget to show up in old clothes, ok? (Key English Test; A2; Portuguese)

oh (EXPRESS EMOTION)
A1 used to express different emotions, such as surprise, disappointment and pleasure, often as a reaction to something someone has said
Dictionary examples:

"Jack can't come after all." "Oh too bad, Amy will be sorry to hear that."

"He ran the Boston marathon last year." "Oh, really? I didn't know that!"

"I'm afraid I can't make it to the party." " Oh well, maybe next time."

Is that for me? Oh, that's so nice of you!

"I'm sorry I forgot to call." "Oh, don't worry."

Learner example:

Oh, that's ok. (Key English Test; A2; Portuguese)

Oh my gosh!
B1 used to emphasize how surprised, angry, shocked, etc. you are
Dictionary example:

Oh my gosh, I'm so happy to see you! When did you get here?

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