O EMMANUEL, God with us, Our King and Lawgiver, the expected of the nations and their savior: COME to save us, O Lord our God.
In the build up of the O Antiphons, lost on the hymn based on them, is the revelation of who it is that we expect to come. O Wisdom, O Lord, O Root of Jesse, O Key of David, O Dawn of the East, O King of the Gentiles and today O God with us, Come!
We await the coming not of an emissary, but of God Himself coming to be with us. All of the other titles have hinted that the coming is not necessarily in the way we expect, because they all, even "Adonai" or Lord hint at the coming of a human savior, again "adonai" being the word that the non-priestly person would have referred to God who the High Priest alone spoke the true name YAHWEH.
In our fallen nature we tend to think we rule ourselves, this was the original temptation that humanity acquiested to in the garden, acknowledging that the God who walked with Adam and Eve in the Garden is our King and Lawgiver is already a step toward salvation.
The next step is to remember this every moment of our lives. He is King, it is His law that we obey for our very life depends entirely on Him.
O Emmanuel, Come!