It is absolutely and without reservation stated in the Old Testament that there is only one God. Period. There are dozens of Old Testament passages which make this a clear as could possibly be. The most famous passage in the entire Hebrew Bible, at least according to many Jews is this: “Hear O Israel, the Lord your God is one Lord” Deut 6:4. So, there is no reasonable argument with the idea that the Old Testament is a monotheistic document.
Yet, Jesus claimed to be God (John 8:58-59) and to be the Son of God (Matthew 16:13-20). John tells us that “In the beginning was the Word. The Word was with God and the Word was God.” (John 1:1) and “The Word of God became flesh and made his dwelling among us. We have seen his glory, the glory of the one and only Son.” (John 1:14).
You point out that this creates a problem. How can God be one, yet two? Actually, if we look at John 14 and John 16 and Matthew 28:19, we see that God is one yet three. Is this hard to grasp? Does this appear at first glance to not even be logical? I say yes to these questions. But this is what Jesus said about himself. This is the same person who fulfilled dozens of messianic prophecies with great precision. This is the same man who walked on water, created bread and fish, healed the deaf and the blind, raised people from the dead and was himself raised from the dead on the third day. Jesus is the fulfillment of the Old Testament messianic expectation. Therefore, I conclude that his statement he is the Son of God is true. To worship Jesus is to worship the one God. To worship the Holy Spirit is to worship the one God. You raise the point that this raises difficult logical questions, and I agree, but this is both what the Old Testament points toward in its many prophecies of the Messiah and it is what the perfect sinless miracle-working Jesus said about himself, so I choose to accept that it is true. You say that the Father did not tell us to worship his Son, but that is not correct. We see more than one example of God speaking and telling people to serve and worship his Son (Matthew 3:17, Matthew 17:5, John 11:41-42 and more). Jesus did say that he was God (John 10:30, John 8:58-59). He did not demand people worship him, naturally, but he certainly did accept worship without reservation (John 20:28 and many more).