Originally Posted by Human_Being
It's easier to see in a definition with example:
/oʊ/ Show Spelled [oh], noun, plural O's.
(used before a name in direct address, especially in solemn or poetic language, to lend earnestness to an appeal): Hear, O Israel!
(used as an expression of surprise, pain, annoyance, longing, gladness, etc.)
the exclamation “O.”
1125–75; Middle English < Old French < Latin ō