"O" or "Oh"?

"O" is a different word with a different meaning from "Oh." 




"O"

- It's a whole word, not a contraction, so no apostrophe is necessary. 
- It's expresses a pleading emotions.
- Used before the name of or a pronoun referring to a person or thing being formally addressed.
Examples:
Please hear our voices , O Mr. President!
O Romeo, where are you?


- An exclamation used in calling or directly addressing a person or personified object; also, as an emotional or impassioned exclamation expressing pain, grief, surprise, desire, fear, etc.
Examples:
O nation of God, rejoice!

"OH" on the other hand expresses more emotion than "O" (surprise or desire) or in response to physical sensation (such as pain)
Examples:
Oh no! The glass is going to break!
Oh, that hurt.
Oh, that does taste good!


- It's also used in direct address.
Examples:
Oh, waiter! Will you come here, please?
Oh sir, I think that's my phone on your purse.


- Another used is to express acknowledgment or understanding of a statement.
Examples:
Oh, now I can see what you mean by that!
Oh, I'm sorry to hear that.



- used during a pause in speaking.
Example:
Their house is about, oh, I'd say four miles from here.