The God of the Holy Bible, is One, "Listen, Israel: The LORD our God, the LORD is One" (Deuteronomy 6:4). This means that the Godhead is One Being. However, it is equally clear from the teaching in the Bible, that there are Three Persons Who are equally called God. While it is true that Jesus Christ is the God of the Old Testament, this does not mean that every time we read "God" or "Lord" in the Old Testament, that it is referring to Jesus Christ. The God of the Bible is not "uni-personal", but "Tri-personal", without being Three Gods.
The Hebrew word for God is "Elohim", which is masculine in gender, and plural in number. Hence we read In Genesis 1:26, "Let US make man in OUR image...". The use of the plural is not because God was addressing the angels, but speaking within the Trinity. In Ecclesiastes 12:1, we read, "Remember now your Creators" (so the Hebrew is in the plural number)
In Genesis chapter 18 we read of Abraham speaking on earth with "Yahweh" (verses 13, 17, 19, 20, etc). In chapter 19 we read, "hen the Lord (Yahweh) rained brimstone and fire on Sodom and Gomorrah, from the Lord (Yahweh) out of the heavens" (verse 24). Where we clearly have the One Lord on earth, Who calls to the Second Lord Who is in heaven!
Psalms 45:6 reads, "Your throne, O God, is forever and ever; A scepter of righteousness is the scepter of Your kingdom", which is addressed to YAHWEH. In Hebrews 1:8, we have God the Father as the Speaker, and He is addressing Jesus Christ, where He says, "But to the Son He says:“Your throne, O God, is forever and ever;A scepter of righteousness is the scepter of Your kingdom".
In Psalm 102 we read of Yahweh, "I said, “O my God, Do not take me away in the midst of my days;
Your years are throughout all generations. Of old You laid the foundation of the earth, And the heavens are the work of Your hands. They will perish, but You will endure; Yes, they will all grow old like a garment; Like a cloak You will change them, And they will be changed. But You are the same, And Your years will have no end.". And again in Hebrews chapter 1, the Father continues addressing Jesus Christ, and says, "And: “You, Lord, in the beginning laid the foundation of the earth, And the heavens are the work of Your hands. They will perish, but You remain;And they will all grow old like a garment;Like a cloak You will fold them up, And they will be changed. But You are the same, And Your years will not fail." (10-12). We have two references equally of God the Father, and God the Son.
In Isaiah 7, we read of the Prophecy of the Birth of the Lord Jesus Christ: " Therefore the Lord Himself will give you a sign: Behold, the virgin shall conceive and bear a Son, and shall call His Name Immanuel" (verse 14). The Name "Immanuel", literally means, "God-with-us", as the Apostle Paul puts it in 1 Timothy 3:16, "God was manifested in the flesh..." (according to the best textual evidence).
In Isaiah 9:6, also speaking of the Lord Jesus Christ, we are told: "For unto us a Child is born, Unto us a Son is given; And the government will be upon His shoulder. And His Name will be called Wonderful, Counselor, Mighty God, Everlasting Father, Prince of Peace". "Mighty God" (el gibbor), which is what God the Father is called in the next chapter of Isaiah, "Mighty God" (10:21). The Title "Everlasting Father", means in the Hebrew, "originator", describing His "eternity".
Isaiah 40:3, "The voice of one crying in the wilderness: “Prepare the way of the Lord; Make straight in the desert A highway for our God.". In the Gospels we are told that this Prophecy was fulfilled in John the Baptist who went before the Lord Jesus Christ. "For this is he who was spoken of by the prophet Isaiah, saying:The voice of one crying in the wilderness:‘Prepare the way of the Lord;Make His paths straight." (Matthew 3:3). Where Luke says, "And he will turn many of the children of Israel to the Lord their God. He will also go before Him in the spirit and power of Elijah, ‘to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just, to make ready a people prepared for the Lord.”" (1:16-17). In verse 17, the words, "He will also go before Him", speak of "the Lord their God".
Isaiah 45:23, "I have sworn by Myself; The word has gone out of My mouth in righteousness,
And shall not return, That to Me every knee shall bow, Every tongue shall take an oath.". Words Spoken by Yahweh, which Paul in his Letter to the Philippians, uses for Jesus Christ, "that at the name of Jesus every knee should bow, of those in heaven, and of those on earth, and of those under the earth, and that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father." (2:10-11)
Jesus Himself says, " Jesus said to them, “Most assuredly, I say to you, before Abraham was, I AM.” (John 8:58). For which the Jews tried to stone Him for alleged "blasphemy" (verse 59). Because they clearly understood the Words of Jesus to be a reference to Exodus 3, when Moses asked God for His Name, to which God replied, "I AM THAT I AM" (3:14), that is "I AM the Eternal One", which is how the Greek Old Testament, some 150 years before the Birth of Jesus Christ, has it, "Εγω ειμι οΩν" "I AM He Who exists", which is the base meaning of the Name, YAHWEH. The Greek of John 8:58 is very interesting, Jesus says, "before Abraham ever existed, I AM", the contrast being between that which "came into being", and that which "always IS"
In 1 Corinthians 10:9, the Apostle Paul writes, "nor let us tempt Christ, as some of them also tempted, and were destroyed by serpents". Words in the Old Testament used for YAHWEH, but applied by Paul to Jesus Christ. The account Paul refers to is Numbers 21:5-6, where we read, " And the people spoke against God and against Moses: “Why have you brought us up out of Egypt to die in the wilderness? For there is no food and no water, and our soul loathes this worthless bread.” So the Lord sent fiery serpents among the people, and they bit the people; and many of the people of Israel died"
There is much more on the Deity of the Lord Jesus Christ in the Old Testament, but this evidence is more than sufficient.