Oi, Oh, Uh-Oh: These are all examples of interjections. They are used in English speech and writing to convey an emotion or feeling. In creative writing they help add context to a scene, and develop the reader's understanding of, and connection with the characters. In everyday English they are commonly used in informal written communications as a substitute for facial expressions and body language.
You can hear more examples and see them being used in our English interjection video lesson online. To help you get to grips with common expressions, sayings, and the correct pronunciation of words, we have produced a series of audio lessons with English native speaker British and American, available to ESO subscribers.