O Clavis David
O Key of David, and Scepture of the House of Israel,
Thou That openest and no man shutteth, and shuttest, and no man openeth:
Come, and lose the prisoner from the prison house,
and him that sitteth in darkness, from the shadow of death.
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 19.
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.