forgot to add: ‘O’ is a quite common form of poetic apostrophe, but even if you don’t use it and just say, “you,” it’s still apostrophe because you’re addressing the person or thing as if they are there and can reply. Example: “Squiggly line in my eye fluid . . . I see you lurking there on the periphery of my vision.” Stewie Griffin uses both ‘O’ and “you” in his poem thus: “O squiggly line in my eye fluid . . . I see you lurking there etc.”

Funny example (from Family Guy) but a good one, no less, to illustrate the device.