O Key of David, Scepter of the house of Israel; that openeth and no man shutteth, and shutteth, and no man openeth: come, and bring forth from the prisionhouse the captive, who sitteth in darkness and in the shadow of death.
From the Blackfriars. Here's the Musica Sacra mp3, with Magnificat. Wikipedia now seems to have an extensive page on the antiphons, including this link at the bottom of the page to some Flash movies and a follow-the-bouncing-ball sort of presentation (that crashed my browser, BTW).
Here's something interesting at Wikipedia that I hadn't known about: two "Antiphons on the Benedictus":
Accompanying the O Antiphons are two Antiphons said at Lauds, on December 21 and 23, also relating to the theme of the upcoming birth of Christ. The first, due to the feast of St Thomas falling on December 21, is traditionally used instead in the commemoration of the feria. With the transference of this feast to July 3 in the revised calendar, these are again used in the Liturgy of the Hours as antiphons. The Monday through Saturday which fall on the days of the O Antiphons also have their own antiphons for the psalms of Lauds, rather than repeating the antiphons of the previous Sunday, as is otherwise done in Advent.
Nolite timere: quinta enim die veniet ad vos Dominus noster.
Fear not, for on the fifth day our Lord will come to you.
Ecce completa sunt omnia, quae dicta sunt per Angelum de Virgine Maria.
Behold, all things are fulfilled, which were spoken by the Angel to the Virgin Mary.
Here's the English version of the chant score of "O Clavis David":