Oh, (answer)

People add "oh" to the beginning of a sentence for lots of different reasons. One way that English speakers use "Oh" is when they have a strong reaction to someone's question. For example:

A: Are you two a couple?

B: Oh, no. No way! We're just friends.

A: Is that a nice neighborhood?

B: Oh, yes. It's one of the best neighborhoods in the city.

So you can use "Oh" when you're really sure about your answer to a question. But a confusing point is that you can also say "Oh" when you're not sure how to answer a question. In that case, you might pause after "Oh":

A: Do you want to invite the Petersons over later this week?

B: Oh... well, maybe. But I'm pretty busy this week.

You have no idea.

"You have no idea" is a way to respond when someone asks you a yes/no question. It means "Yes, very much." For example:

A: How was Jamaica? Was it fun?

B: Oh, you have no idea!

The second speaker wants to say that Jamaica was very, very fun. Here's another example:

A: Was it difficult, completing your dissertation?

B: You have no idea. I thought that I was going to go insane! I was so glad when it was over.