Like all the other 'O' antiphons 'O Emmanuel' the 'O' antiphon for 23 December is based upon the Book of Isaiah, in this case Isaiah 7:14:
Isaiah 7:14 Propter hoc dabit Dominus ipse vobis signum : ecce virgo concipiet, et pariet filium, et vocabitur nomen ejus Emmanuel. (Therefore the Lord himself shall give you a sign. Behold a virgin shall conceive, and bear a son and his name shall be called Emmanuel).
It's sung below Peter Morton (tenor) and the choir of St John's College Choir Cambridge, conducted by David Hill.