O Rex Gentium
O King of the Gentiles, and their Desire,
the Corner-stone, Who madest both one:
Come and save man,
whom Thou hast made out of the dust of the earth.
From The Greater Antiphons At Evensong During Eight Days Before Christmas from The Salisbury Antiphonary, edited by John Mason Neale and Thomas Helmore, in Hymnal Noted: Parts I and II (London: Novello, 1856). Neal and Helmore directed that the antiphons start a day earlier than is usual, so this was their antiphon for Evensong on December 21.
The “O Antiphons” refer to the seven antiphons that are recited (or chanted) preceding the Magnificat (the Song of Mary) during Vespers of the Liturgy of the Hours. They cover the special period of Advent preparation known as the “Octave before Christmas”, December 17 through 23, with December 24 being Christmas Eve and Vespers for that evening being for the Christmas Vigil. The exact origin of the “O Antiphons” is not known.