Paul certainly regarded Jesus as the "Lord God" of the OT (where kurios "Lord" replaces YHWH in the LXX). In the epistle of Romans alone, he twice quotes passages referring to YHWH that he applies explicitly to Jesus:
Joel 2:26-32 LXX: "And eating you shall eat and be satisfied and praise the name of the Lord your God in regard to the marvelous things he has done with you. And my people shall never again be put to shame. You shall know that I am in the midst of Israel and I am the Lord your God, and there is none other but me. And my people shall never again be put to shame. And it shall be after these things, I will pour out my spirit on al flesh and your sons and your daughters shall prophesy and your old men shall dream dreams, and your young men shall see visions. Even on the male and female slaves, in those days I will pour out my spirit. I will give portents in the sky and on earth, blood and fire and the vapor of smoke. The sun shall be turned to darkness and the moon to blood, before the great and notable day of the Lord comes. And it shall be that everyone (pas) who calls (epikalesétai) on the name of the Lord (kuriou) shall be saved, because in Mount Zion and in Jerusalem there shall be one who escapes as the Lord has said, and people who have good news announced to them, whom the Lord has called".
Romans 10:9-13: "That if you confess (homologésés) with your mouth, 'Jesus is Lord (kurios),' and believe in your heart that God raised him from the dead, you will be saved (sóthésé). For it is with your heart that you believe and are justified, and it is with your mouth that you confess (homologeiti) for salvation (sótérian). As the Scripture says, 'Anyone who trusts in him will never be put to shame'. For there is no difference between Jew and Gentile -- he is the same Lord (kurios) of all (pantón) and richly blesses all (pantas) who call (epikaloumenos) on him for (gar, indicating a logical relation), 'Everyone (pas) who calls (epikalesétai) on the name of the Lord (kuriou) will be saved (sóthésetai)".
Here Paul uses Joel 2:32 LXX as a prooftext supporting his argument that the confession of Jesus as Lord brings salvation. Notice the use of gar to connect the OT intertext to his argument which in the previous sentence he referred to Jesus as the same "Lord" (kurios) of all (pantón) and who blesses all (pantas) who call (epikaloumenos) on him. This uses the original language of intertext (i.e. the words pas, epikalesétai, and understanding sóthésetai as a form of blessing) in his own argument, which supports his main assertion that if you confess that Jesus is Lord (kurios), you will be saved (sóthésé). This latter word also reflects the use of Joel 2:32 LXX. Throughout kurios is the central word, for it is part of the declaration that brings salvation according to the scripture. Jesus is thus clearly identified as the "Lord" and "Lord your God" of the text in Joel (which is YHWH throughout the original Hebrew and kurios in the Greek).
Isaiah 45:18-25 LXX: "Thus says the Lord who made heaven -- this is the God who displayed the earth and made it, he himself marked its limits ... -- I am, and there is no other. I have not spoken in secret nor in a dark place of the earth... I am, I am the Lord, speaking righteousness and declaring truth... I am God and there is no other besides me; there is no righteous one or Savior expect me. Turn to me and you shall be saved, you who are from the end of the earth. I am God and there is no other. By myself I swear, 'Verily righteousness shall go forth from my mouth, my words shall not be turned back, because to me every knee shall bow and every tongue acknowledge God saying, "Righteousness and glory shall come to him and all who separate themselves shall be ashamed." ' By the Lord shall they be justified, and all the offspring of the sons of Israel shall be glorified in God".Romans 14:8-11: "If we live (zómen), we live (zómen) to the Lord (kurió); and if we die (apothnéskómen), we die (apothnéskómen) to the Lord (kurió). So, whether we live or die (zómen te apothnéskómen), we belong to the Lord (kuriou). For this very reason (eis touto, indicating the logical relation), Christ died (apethanen) and returned to life (ezésen) so that he might be the Lord (kurieusé, notice that this is a verb) of both the dead and the living (i.e. everyone). You, then, why do you judge your brother? Or why do you look down on your brother? For we will all (pantes) stand before God's judgment seat. It is written: 'As surely as I live (zó),' says the Lord (kurios), 'every (pan) knee will bow before me; every (pasa) tongue will confess to God' ".
Here Paul quotes Isaiah 45:23 LXX in support of his argument that Christ (who died and returned to life) is the Lord over both the living and the dead. There is a distinct connection between Christ's lordship over the "living and dead" who shall "all" stand before the judgment seat and the lordship of the "Lord" (kurios) over "every knee and every tongue", as well as between Christ "coming to life" in order to become Lord (kurieuó) and Christians who "live for the Lord" (cf. the same verb zó in the quotation in which kurios declares that he "lives"). The "Lord" that Christians live and die for is Christ (who is Lord for the dead and living) and this the same "Lord" who says that every knee will bow before him. This reference to knees bowing before him relates to the reference of all standing before God's judgment seat, just as it relates to the reference to Christ being Lord of the dead and the living. Christ is the same "Lord" in the passage quoted from Isaiah.