1.used when you start telling someone something, for example something that you have just remembered; used at the beginning of a reply for showing that someone has just reminded you of something
2.used for expressing an emotion such as surprise, anger, or happiness; used in expressions such as “Oh God!,” “Oh dear!,” “Oh no!”, and “Oh my!” for showing an emotion such as surprise, fear, or disappointment. A lot of people may say “Oh God” , but it offends some people.
3.used in expressions such as “Oh I see” and “Oh right” for showing that you now understand something
4.used for accepting someone’s answer to your question
5.used when you pause because you are thinking about what you are going to say
6.used for showing that you did not know something
7.used when telling a story for introducing the words that you or someone else has said