In John 14:15, Jesus says, "If you love me, keep my commandments." What commandments would that be? Is Jesus the God of the Old Testament?
The same commandments Jesus told the apostles to teach: "And Jesus came and spoke to them, saying, "All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age." Amen" (Matthew 28:18-20). Therefore all that is contained in the New Testament, including the apostles teachings, are the commandments of Jesus. "If anyone thinks himself to be a prophet or spiritual, let him acknowledge that the things which I write to you are the commandments of the Lord" (I Corinthians 14:37).
Jesus was with God and is God. "In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God" (John 1:1-2). Therefore, when the Old Testament mentions that God did or said something, that would include Jesus as well as the Father and the Spirit. However, the commands that Jesus was referring to were his commands as the Father turned all thing over to the Son. "For "He has put all things under His feet." But when He says "all things are put under Him," it is evident that He who put all things under Him is excepted. Now when all things are made subject to Him, then the Son Himself will also be subject to Him who put all things under Him, that God may be all in all" (I Corinthians 15:27-28). From this we conclude that during the Old Testament time God, the Father, was dominate. In the Christian age God, the Son, is dominant but at the end of the age the authority will return to the Father. "But I want you to know that the head of every man is Christ, the head of woman is man, and the head of Christ is God" (I Corinthians 11:3).
The commands, the covenant between God and man, is different under Christ that it was in the Old Testament. "Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah -- not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the LORD. But this is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. No more shall every man teach his neighbor, and every man his brother, saying, 'Know the LORD,' for they all shall know Me, from the least of them to the greatest of them, says the LORD. For I will forgive their iniquity, and their sin I will remember no more" (Jeremiah 31:31-34). "In that He says, "A new covenant," He has made the first obsolete. Now what is becoming obsolete and growing old is ready to vanish away" (Hebrews 8:13).