The sixth great “O” is: O Rex gentium, et desideratus earum, lapisque angularis, qui facis utraque unum: veni, et salva hominem, quem de limo formasti.

English: O King of the gentiles and their desired One, the cornerstone that makes both one: come, and deliver man, whom you formed out of the dust of the earth.

It reflects the following prophecies and Scripture:

O King of the gentiles and their desired One,

• And they sing the song of Moses the servant of God, and the song of the Lamb, saying, Great and marvellous are thy works, Lord God Almighty; just and true are thy ways, thou King of saints. Revelation 15:3 Most probably the song of deliverance after the passage of the Red Sea (Exodus 15.), to which this bears a general resemblance. Moses is called the “servant of God” in Exodus 14:31 and elsewhere. The song of Moses is also the song of the Lamb; the Old Testament and the New Testament Churches are one. Saying, Great and marvellous are thy works, Lord God Almighty (cf. Exodus 15:7, “And in the greatness of thine excellency thou hast overthrown them;” also Psalm 111:2; Psalm 139:14). This song, like that in Revelation 4:8, is addressed to the “Lord God Almighty.” Christ is in this song addressed as a divine person, as Lord of all, God over all, blessed for ever, the Almighty God, as His works declare Him to be; His works of creation, providence, and redemption, which are all great and marvellous, particularly the accomplishment of the glorious things spoken of His church, and the destruction of His enemies, which are here designed (see O Adonai and O Clavis David).

the cornerstone…

• Therefore thus saith the Lord GOD, Behold, I lay in Zion for a foundation a stone, a tried stone, a precious corner stone, a sure foundation: he that believeth shall not make haste. Isaiah 28:16 [comp. The stone which the builders refused is become the head stone of the corner. Psalm 118:22] In contrast with the insecure refuge and false ground of confidence whereon the nobles relied, the prophet puts forward the one sure “Rock” on which complete dependence may be placed – which he declares that Jehovah is laying, or “has laid,” in Zion as a tried stone, a precious corner stone, a sure foundation. The imagery is, no doubt, drawn from the practice of Oriental kings, and notably Solomon, to employ foundation-stones of enormous size and weight at the corners of buildings. Some of those uncovered at the corners of Solomon’s temple by the Palestine Exploration Fund are more than thirty-eight feet long, and weigh above a hundred tons (see ‘Our Work in Palestine,’ pp. 38, 115). But the reference cannot, of course, be to the material structure of the temple as Israel’s true refuge. Rather, Jehovah himself would seem to be the Rock (Isaiah 26:4; Isaiah 30:29, etc.) intended; and hence the application to Christ by the writers of the New Testament (Romans 9:33; Ephesians 2:20; 1 Peter 2:6-8) was natural and easy.

…that makes both one:

• “Jesus saith unto them, Did ye never read in the scriptures, The stone which the builders rejected, the same is become the head of the corner: this is the Lord’s doing, and it is marvellous in our eyes?” Matthew 21:42 [Mark 12:10; Luke 20:17] By the Old Testament saints, and by saints in all ages, who have ventured their souls on Him, and laid the whole stress of their salvation upon Him, and have been saved by Him; and by Satan, and his principalities and powers, by his temptations of Him in the wilderness, and by his attacks upon Him in the garden, and on the cross, and found Him to be an immovable stone, and were broken by Him; and by His divine Father, who tried His faithfulness by trusting Him with all His elect, and the salvation of them; and His great strength, by laying upon Him all their sins, and the punishment due unto them. Some render it, “a stone of trial”, or “a trying stone” by which men are tried, and discovered to be what they are, whether believers or unbelievers, sincere Christians or hypocrites; which may be known by their conduct and behaviour to Christ; if they come to Him as a living stone, and He is precious to them, they are true believers; but if He is to them a stone of stumbling, and a rock of offence, they are unbelievers, and reprobate persons, 1 Peter 2:4,
• This is the stone which was set at nought of you builders, which is become the head of the corner. Acts 4:11 So the Apostles preach themselves that Christ is the foundation of all their ministry, so that the churches “…are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone” Ephesians 2:20

come, and deliver man, whom you formed out of the dust of the earth.

• So the Church expresses the prayer of the redeemed who recognise Christ as the “Word” i.e. the “logos”, the “Ruach Elohim” the Creator with God of the world, see O Sapientia.