The big O
No, not that big O.
O and oh were once no more than variant spellings, but a differentiation in meaning has developed.
O—you can call it the vocative O if you like Latin terms of grammar—is the form used in direct address to a person, a deity, an audience. “O say, can you see. ...” Reserve it for such direct address.
Oh can be a mere interjection, as in “Oh, I forgot to mention. ...” But it is more often an exclamation, indicating surprise, dismay, disgust, excitement, joy, or some other emotion. That is the oh abbreviated in the adolescent text abbreviation OMG. There is speculation that it is often a person’s next to last word.