O Lord, ✠ thou wilt open my lips.

David in Prayer. Willem Vrelant. (Flemish, early 1460s)
Getty Centre [Public Domain]

V. Dómine, lábia ✠ (Sign of cross over lips) mea apéries.
V. O Lord, ✠ open thou my lips.

R. Et os meum annuntiábit laudem tuam.
R. And my mouth shall declare thy praise.

These words come from verse 17 of Psalm 50 and open Matins every day. In the literal sense, King David is asking for permission to praise God worthily again after his sin. In the particular case of the monk, these are the first words said aloud in the darkness after the nightly great silence.  For all of us, the night is the dark night of our sin, recalling that sin is the death of the soul. We pray that we may arise to the light of a new day, contrite and refreshed in the hope of being allowed to sing God's praises once more.

Totus tuus ego sum 
Et omnia mea tua sunt;
Tecum tutus semper sum:
Ad Jesum per Mariam

Here is the context of the words in verse 17:

[16] Libera me de sanguinibus, Deus, Deus salutis meae, et exsultabit lingua mea justitiam tuam.
Deliver me from blood, O God, thou God of my salvation: and my tongue shall extol thy justice.
[17] Domine, labia mea aperies, et os meum annuntiabit laudem tuam.
O Lord, thou wilt open my lips: and my mouth shall declare thy praise.
[18] Quoniam si voluisses sacrificium, dedissem utique; holocaustis non delectaberis.
For if thou hadst desired sacrifice, I would indeed have given it: with burnt offerings thou wilt not be delighted.
[19] Sacrificium Deo spiritus contribulatus; cor contritum et humiliatum, Deus, non despicies.
A sacrifice to God is an afflicted spirit: a contrite and humbled heart, O God, thou wilt not despise.

St Robert Bellarmine's commentary

Having prayed shortly before for his sins to be washed away, and having promised that he would teach sinners the ways of the Lord, David now prays to be freed from the punishment which Urias’s blood, unjustly spilt, called for, and promises to praise God’s justice. “Deliver me;” save me from the voice of Urias’s blood, which, unjustly spilled by me, cries out to thee and calls for vengeance; “Deliver me,” for he fancied he saw the blood, like a soldier in arms, staring him in the face; and, therefore, with great propriety, he adds, “O God, the God of my salvation;” for to deliver from imminent danger is the province of a Saviour; and this, too, is a reason for his adding, “and my tongue shall extol thy justice;” for true deliverance and salvation was then had through the merits of Christ in prospective, as the same is had now through the same merits as of the past. The merits of Christ have in them the very essence of justice, and deserve the most unbounded praises both of lips and of heart on our part.

The consequence of the perfect justification and salvation of the sinner is, that his lips, which were wont to praise God, but were closed by sin, through his pardon should be opened again to praise and thank his Redeemer. He, therefore, says, “O Lord, thou wilt open any lips,” by forgiving and pardoning my sins, and restoring my joy and confidence; you will open my lips, and then “my mouth shall declare thy praise,” by proclaiming your mercy and justice, not only to the present but to all future ages.

He assigns a reason for offering the sacrifice of praise, because sacrifices of cattle are not pleasing to God; as if he said, “My mouth shall announce thy praise,” because I know you to prefer such sacrifice to that of brute animals; and if such sacrifices were pleasing to you, I would not hesitate in offering them. It is not to be inferred from this, that sacrifices of brute animals were in no respect pleasing to God, when it is clear, from the book of Leviticus, that they were instituted and ordered to be offered by him; but they are said to be of no value essentially, as if the slaughter of cattle were, in itself, a thing agreeable, or useful, or necessary to God. They are also said to be of no value in comparison with the sacrifice of the Eucharist, as appears from Malachias 1, where the old sacrifices, it is said, will cease, when “The clean oblation will be offered in all nations.” Sacrifices are also said to be of no value when they are offered by sinners, as we have in Isaias 1, “Obedience being more pleasing to God than the offering of victims.” Finally, sacrifices are said to be of no value as regards the expiation of sin; for, as the Apostle says, “It is impossible that sins could be taken away by the blood of bulls and goats;” and it is in such sense that David says here, “If thou hadst desired sacrifice,” for the remission of my sins, “I would indeed have given it;” but because “with burnt offerings thou wilt not be delighted,” so as to forgive me my sins through them, therefore “My mouth shall declare thy praise;” for, as we said in the explanation of the last Psalm, such sacrifice is the one most acceptable to God, being lighted on the altar of the heart with the fire of charity.

He explains more fully how acceptable to God is the sacrifice of praise; that sacrifice that springs from a contrite and humbled heart, when man, acknowledging his own misery and God’s mercy, humbles himself before his power, attributing all honour and glory to him, and confusion and disgrace to himself, as we read in Daniel 9, “Justice to thee, O Lord, but to us confusion of face;” and a little further on, “To us, O Lord, confusion of face, to our kings, our princes, and our fathers who have sinned, but to you, our Lord God, mercy and propitiation.” The expressions, “afflicted spirit” and “contrite heart,” are the same, and the one Hebrew expression is only given for both, but the interpreter chose to vary the words, and the meaning is the same. The spirit is said to be afflicted when the soul is affected with grief, and thus placed in trouble, by reason of the sin committed against God; so also, the heart is said to be contrite when the soul, full of grief for the sin committed, is, as it were, torn asunder, and reduced into powder, from its strong hardness and insensibility. Such contrition is the sacrifice most acceptable to God, for as well as he is offended by our sins, he is appeased by our repentance; and very properly is now added, “A contrite and humbled heart, O Lord, thou wilt not despise;” for God despises the proud, and resists them; but to the humble (who willingly submit to him) he always gives his grace, James 4.

The works of justice that please God as true spiritual sacrifices are the effect of justification, according to the Apostle, Heb. 13, “And do not forget to do good, and to impart, for by such sacrifices God’s favour is obtained;” and 1 Pet. 2, “Offer up spiritual sacrifices acceptable to God by Jesus Christ.”[1]—“Then,” when I shall have been thoroughly renewed and justified, “shalt thou accept the sacrifice of justice;” all the good works of mine and my people, “oblations and whole burnt offerings.” All which good works will be so many spiritual oblations, so many spiritual holocausts. Spiritual oblations are the offering of one’s substance or property in alms for the love of God; and spiritual holocausts are the dedication of one’s self entirely to do God’s will and commands, according to Rom. 11, “I beseech you therefore brethren, by the mercy of God, that you present your bodies a living sacrifice, holy, pleasing to God, your reasonable service.”—“Then shall they lay calves upon thy altar.” When it shall be seen that such sacrifices of justice are the most acceptable to you, people will vie with each other in loading your altar, not with the ordinary sacrifices, but with the most precious; for that of the calf was considered the sacrifice most valuable; and thus the “laying calves upon the altar” means the offering of works of the most perfect justice to the Lord God.

[1]  Compare these observations to the words recommended at Fatima in 1917 to the little seers when offering sacrifices:

O my Jesus, it is for love of Thee, for the conversion of sinners and 
in reparation for sins committed against the Immaculate Heart of Mary.

Totus tuus ego sum 
Et omnia mea tua sunt;
Tecum tutus semper sum:
Ad Jesum per Mariam