In the medieval Sarum (Anglican) use they began on Dec. 16, adding on Dec. 23, O Virgo virginum (O Virgin of virgins). The Great O Antiphons have been adapted as a popular Advent hymn, "O come, O come, Emmanuel" (Hymn 56), whose verses may be used as antiphons for the Magnificat on the appropriate days.
Today's O Antiphon (from Hymn 56):
O come, thou Wisdom from on high,
who orderest all things mightily;
to us the path of knowledge show,
and teach us in her ways to go.