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.