Oh, OH or Oh! is an interjection, proclaiming surprise (emotion). That's at least the official explanation in Wikipedia.
I don't know if there are only three Ohs or more because you can use it in so many different situations.
1. I go in a restroom, open the toilet door and a woman is sitting on the throne, I quickly close the door in horror and scream "Oh".
2. Looking out of the window, it rains and I say "Oh" I have to take my umbrella.
3. I see little kittens in a basket and a deep "Oooh" eventually also Awww comes out of my throat.
4. In a restaurant I kick over the bottle of water and exclaim "Oh" (I am sorry)
5. Somebody rings at the door, I open and say "Oh ... it's only you !"
6. Standing on a balcony on the third floor I admire a sunset and "Oh, is that beautiful" comes out of my mouth.
7. Somebody called my name, I answered and then quickly added, Oh, I am sorry I thought you were Mary.
8. Then occasionally you say "Oh ! you cut your hair"
9. Oh ! you got divorced I didn't know !
10. Oh sorry, I thought it was my chair.... when I took a chair and the person who wanted it too suddenly sat on the floor.
Oh, I think I could continue but oh, it's late my bed calls me !
Linking to Linda G. Hills SOCS - OH