Gregory DiPippo at New Liturgical Movement has a wonderful essay about the so-called O antiphons, which started being sung at vespers (at the Magnificat) on December 17. These antiphons survived the post-conciliar liturgical reforms and are said even in the Liturgia Horarum. A brief selection, about the antiphon sung today, O Adonai:
“O Adonai” speaks of Christ as the one who appeared to Moses in the burning bush, and gave him the Law on Mount Sinai; “Adonai”, Hebrew for “My Lord”, is the word which Jews, when reading the Bible, say in place of the Divine Name YHWH that was revealed to Moses in Exodus 3. The prayer to “come to redeem us with arm extended” refers to God’s own words when speaking to Moses in Exodus 6, 6, “I am the Lord who will bring you out from the work-prison of the Egyptians, and will deliver you from bondage: and redeem you with a high arm, and great judgments,” as well as the canticle which Moses sings after the crossing of the Red Sea, “Let fear and dread fall upon them, (i.e. upon the Egyptians) in the greatness of thy arm.” (Exod. 15, 16)
(Hyperlink in original) Read the whole thing there.