actually, everyone has it partially correct – i’m surprised this isn’t taught in basic english grammar anymore – but i digress.
the use of ‘o’ is a recognized poetic and literary device (always capitalizd) and called “apostrophe.” the device is used to address a person or a thing as if there (whether it’s yours or not, such as kimberly grey’s body). so, using, “O Body!” could refer to your body (in my case, lamenting it) or someone else’s (in that case, usually extolling it – haha that’s just me). but the thing to remember is you use “O” to address something or person as if it is there with you as you wax poetic.
…and not to be confused with an actual ‘ apostrophe, which is punctuation.