There is a little-known fact about the order of these ancient O Antiphons. This is not apparent in English, but it can be seen clearly in the official language of the Roman Catholic Church: Latin. The Latin versions of each of the titles of the Messiah: Sapientia (Wisdom), Adonai (Lord), Radix (Root), Clavis (Key), Oriens (Dawn), Rex (King), and Emmanuel (Emmanuel). Take the first letters of each of the titles and write them backwards, thus counting down the days of the feast: EROCRAS or "ero cras." This means in Latin "I will be (here) tomorrow".
The song "O Come, O Come, Emmanuel" is simply a slightly modified sung version of the seven O Antiphons.