O Emmanuel

O Emmanuel, Rex et legifer noster, exspectatio gentium, et Salvator earum, veni ad salvandum nos Domine Deus noster.
 
O Emmanuel, our King and Lawgiver, the Desire of all nations, and their Salvation: come and save us, O Lord our God. 
 
'Emmanuel' derives from Isaiah 7:14,
'Ecce virgo concipiet, et pariet filium,
Et vocabitur nomen eius Emmanuel'
'Behold, a virgin shall conceive, and bear a son,
And his name shall be called Emmanuel.'

This is referred to the birth of Christ in St Matthew's Gospel:

'Hoc autem totum factum est, ut adimpleretur quod dictum est a Domino per prophetam dicentam: Ecco virgo in utero habebit, et pariet filium, et vocabunt nomen eius Emmanuel,
quod est interpretatum Nobiscum Deus.'
'Now all this was done, that it might be fulfilled which was said by the Lord through the prophet,
saying: Behold, a virgin shall have a son in her womb, and bear him, and they shall call his name Emmanuel, which is, being translated, God with us.' (Matthew 1:22-23).

Emmanuel, 'God with us', is perhaps the most important title in the series.

'Rex', 'King' is a title often applied to Christ in the New Testament, e.g. at Matthew 2:2, 'Ubi est qui natus est rex Iudaeorum?' 'Where is he that has been born King of the Jews?' Or the title placed on the cross: 'Hic est Iesus rex Iudaeorum' 'This is Jesus, King of the Jews' (Matthew 27:37).

'Legifer', 'lawgiver' equates Jesus with Moses who gave the law to the Israelites on Mount Sinai. Jesus is portrayed as giving a new law, e.g. in his delivery of the Sermon on the Mount, Matthew 5-7. Or cf. John 13:34, 'Mandatum novum do vobis: ut diligatis invicem, sicut dilexi vos' - 'A new Commandment I give you, that you should love one another, as I have loved you.' ['Mandatum' here gives us 'Maundy' as in Maundy Thursday, the day of the Mandate].

'Exspectatio gentium' has already been mentioned with reference to 'O Clavis David'. It derives from Genesis 49:10,

Non aufertur sceptrum de Iuda,
Et dux de femore eius,
Donec veniat qui mittendus est,
Et ipse erit expectatio gentium.

'The sceptre shall not be taken away from Judah,
nor the leader from his thigh,
until he comes who is to be sent,
and he will be the expectation of the nations.'

'Salvator', 'Saviour', is applied regularly in the Old Testament to God, and equally regularly in the New Testamen to Jesus. The equation is made explicit in the last words of our antiphon, 'veni ad salvandum nos Domine Deus noster' - 'Come and save us, O Lord our God'. 
 
Fr East adds this comment "In England, there was an eighth antiphon, 'O virgo virginum', 'O virgin of virgins', applied to Mary; and example of English exhuberance spoiling the careful and spare patterning of the Roman liturgy."