O Radix Jesse
O Root of Jesse, Who standest for an ensign of the people,
at Whom Kings shall shut their mouths,
unto Whom the Gentiles shall pray:
Come and deliver us, and tarry not.
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 18.
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.