O, oh

   O is confined almost exclusively to religious and poetic contexts. By convention it is always capitalized and never followed by punctuation. Thus if rendering a prayer you would write: "O Lord, who has drawn over weary day the restful veil of night...." Oh is used in all other senses and is normally set off with a comma or commas: "We hunted for him for, oh, seven hours"; "Oh, I think it was a green car." If a sentence employs a reverential word but is not actually reverential in intent, use oh, as in "Oh, god, I think she's spotted me" or "Oh, lord, I don't remember his name."