Today is the eighth day before Christmas, we're in the final countdown to Christmas. In these the final days of preparation for the celebration of Christmas we are given a greater opportunity to reflect. If you attend Daily Mass you will hear them as the verse for the Gospel Acclamation, in the Liturgy of the Hours they are the anthiphon before and after the Magnificat for Vespers, you can also just pray them as part of your devotions during Advent maybe with the lighting of the Advent wreath. The O Antiphons not only highlight titles for the Messiah but each one also refers to the prophecy of Isaiah of the coming of the Messiah.
O Sapientia (Wisdom). Is 11:2-3, Is 28:29
O Adonai (Lord). Is 11:4-5, Is 33:22
O Radix Jesse (Root of Jesse). Is 11:1, Is 11:10
O Clavis David (Key of David). Is 9:6, Is 22:22
O Oriens (Dayspring or Radiant Dawn). Is 9:1
O Rex Gentium (King of Nations). Is 2:4, Is 9:5
O Emmanuel (God is with us). Is 7:14
Fun fact, the first letters of the titles read in reverse order form the Latin word Ero Cras which means "I will be with you tomorrow".