"Oh" is a word that usually shows suprise. You use it when you find out some new information that you didn't expect:

Oh, you two know each other? I had no idea!

But in polite conversation, people sometimes use "Oh" in a subtle way. You can use it to appear surprised when someone does something nice for you. This makes you seem humble. For example, you can respond to a compliment using "Oh":

A: You're a good writer.

B: Oh, thank you!

Or if someone gives you a gift, you can seem surprised:

Oh, you got me something for my birthday? You shouldn't have! Thank you so much!