A set of antiphons, called "O" from the first word in each of them. They are in Latin, and go back at least as far as the ninth century, when they are reported by Amalarius of Metz. They are prayers for the soon advent of Jesus at Christmas, addressing Him by titles used in the Old Testament, and are traditionally sung December 17 through 23 (one each night, if I understand correctly).

The normal order is as follows:

  1. O Sapientia (O Wisdom)
  2. O Adonai (O Adonai)
  3. O Radix Jesse (O Root of Jesse)
  4. O Clavis David (O Key of David)
  5. O Oriens (O Dawn)
  6. O Rex Gentium (O King of Nations)
  7. O Emmanuel (O Emmanuel)

The (non-O) initials of these, taken in reverse, spell Ero Cras, meaning roughly "I will come tomorrow".

The O Antiphons were the basis for the Latin hymn Veni Veni Emmanuel written in the 18th century, and therefore also the English song O Come O Come Emmanuel.

(Update, 14 December 2K) Apparently there are and have been other O Antiphons around, although these are the 'main' ones. The Catholic Encyclopedia lists 'O Virgo Virginum' for December 18 alongside 'O Adonai'. Also there was 'O Gabriel', later replaced by 'O Thoma Didyme' for December 21.

Some medieval churches had twelve greater antiphons, adding to the above (1) "O Rex Pacifice", (2) "O Mundi Domina", (3) "O Hierusalem", addressed respectively to Our Lord, Our Lady, and Jerusalem.

from http://www.newadvent.org/cathen/11173b.htm