The O Antiphons are ancient Advent hymns which are traditionally part of Evening Prayer on the 8 days before Christmas (the last one, O Virgo Virginum, is newer than the rest). The older ones are inspired by the verses of O Come O Come Emanuel. Traditionally our choir has sung selections from the O Antiphons each Sunday of Advent, before the start of mass; our arrangement is in English with 4-part harmony, from Anglican hymnity.
Wikipedia has a good article on the subject, and Father Z provides blog posts on each antiphon besides O Virgo Virginum which can help you medidate on their significance to the story of the Incarnation.
