HAMLET: O, I die, Horatio!
The potent poison quite o’ercrows my spirit.
I cannot live to hear the news from England.
But I do prophesy th’ election lights
On Fortinbras; he has my dying voice.
So tell him, with th’ occurrents, more and less,
Which have solicited — the rest is silence.
[Dies.]

William Shakespeare (1564-1616) English dramatist and poet

Hamlet, Act 5, sc. 2, l. 389ff (5.2.389-395) (c. 1600)
    (Source)

Just before Fortinbras and the English ambassadors enter.

In the First Folio, Hamlet moans, "O, O, O, O!" just before dying.