The figure or symbol O has been used in reference to emptiness or absence (i.e. basically our number 0) since circa 976 according to Wikipedia.
The actual letter has served the same purpose since circa 1425 according to the 0ED.
The letter has been used more broadly in reference to absence, nothingness or naught (nought) since as late as 1608 (and I’m sure much much earlier), viz. in a play entitled King Lear written by a playwright called William Shakespeare, in which the line ‘Thou art an O without a figure, I am better then thou art now, I am a foole, thou art nothing’ is spoken (I.iv.174).