The O Antiphons are a beloved ancient Advent tradition. Begun in the early Middle Ages, they were chanted in Latin by monks during the Divine Office. An antiphon is a response sung during liturgical prayers such as Mass or Vespers (evening prayer). During the last week of Advent, the O Antiphons are added to the vespers sung before the Magnificat to heighten the anticipation of “the coming”. The ‘O’s reflect the longing of a sinful mankind for our Savior, calling on Christ with titles from the Old Testament, drawing ever closer in time to His coming. The popular Advent hymn “O Come, O Come, Emmanuel” is based on the O Antiphons.

Even the arrangement of the O Antiphons has special meaning. The first letter in each, from last to first, spells in Latin “ero cras”, which means “Tomorrow, I will come”.

Other symbolism in the illustration appears in the decorative flourishes. On the “O Antiphon” shield is a cluster of holly leaves and berries. The evergreen leaves traditionally represent the divinity of Christ. The sharp points on the leaves and the red berries, reminiscent of the crown of thorns and drops of blood, are striking symbols of Christ’s humanity. The fruit of the plant stemming from the root of Jesse is wheat and grapes, the ingredients for the bread and wine which would become the body and blood of Christ.

 
O Sapientia - The dove is an ancient symbol of the wisdom of God.
O Wisdom (Eccl 24: 5), you came forth from the mouth of the Most High (Sir 24: 30), and reaching from beginning to end, you ordered all things mightily and sweetly (Wis 8: 1). Come, and teach us the way of prudence (Isa 40: 14).
 
O Adonai - God appeared to Moses in the burning bush.
O Lord and Ruler (Ex 6: 13) of the house of Israel (Mt 2: 6), you appeared to Moses in the fire of the burning bush (Ex 3: 2), and on Mount Sinai gave him your Law (Ex 20). Come, and with outstretched arm redeem us (Jer 32: 21).
 
O Radix Jesse - The root and fruitful plant represent the lineage of Our Savior.
O Root of Jesse, you stand for the ensign of all mankind (Isa 11: 10); before you kings shall keep silence and to you all nations shall have recourse (Isa 52: 15). Come, save us, and do not delay (Hab 2: 3).
 
O Clavis David - The key is a symbol of the Davidic Kings’ authority, of which Jesus is heir.
O Key of David (Apoc 3: 7) Scepter of the house of Israel, you open and no man closes; you close and no man opens (Isa 22: 22). Come, and deliver him from the chains of prison who sits in darkness and in the shadow of death (Ps 107: 10)
 
O Oriens - The rising sun symbolizes the coming of the light of God.
O Rising Dawn (Zech 6: 12), Radiance of the Light eternal (Hab 3: 4) and Sun of Justice (Mal 3: 20); come, enlighten those who sit in darkness & the shadow of death (Ps 107: 10; Lk 1: 78).
 
O Rex Gentium - The crown represents Christ’s kingship over the Jews and all mankind.
O King of the Gentiles (Hag 2: 8), Desired of all, you are the cornerstone that binds two into one (Eph 2: 20). Come, and save poor man whom you fashion out of clay (Gen 2: 7).
 
O Emmanuel - The infant Jesus is the Incarnation of the Son of God, true God and true man.
O God with Us (Isa 7: 14; 8: 8), our King and Lawgiver (Gen 49:10; cf. Ezek 21: 32), the Expected of the nations and their Savior (Isa 33: 22): Come, and save us, O Lord our God.
 
 
 
Sources:
http://www.catholicculture.org/lit/activities/view.cfm?id=958
http://www.catholiceducation.org/articles/religion/re0374.html