In today’s English, oh is an interjection used to express a range of emotions, including pain, sorrow, hesitation, and recognition. Most people will never have use for O, which is used in poetic apostrophe, usually in classical addresses, always preceding the name of or pronoun representing the person being formally addressed.
Let’s call it, oh, I don’t know, an appreciation. [GoErie.com]
That spot is good for, oh, about $11 million during the first two guaranteed season. [Arizona Daily Star]
Oh, I know my kids aren’t mini-saints or perfect angels. [Whittier Daily News]
O is always capitalized, it always immediately precedes the person being addressed, and in modern use it is usually employed to create an archaic tone. Here are a few examples:
Then the angel of Yahweh replied, “O Yahweh of Armies, how long will you not have mercy on Jerusalem and on the cities of Judah, against which you have had indignation these seventy years?” [The World English Bible]
Tell me, O Muse, of that ingenious hero who travelled far and wide after he had sacked the famous town of Troy. [The Odyssey]
O mother of flames, / You who have kept the fire burning! / Lo, I am helpless! / Would God they had taken me with them! [William Carlos Williams, “Crude Lament”]