There are not a lot of OT writings which unambiguously point to the Messiah/Christ being God, but I can think of a couple. One is Isaiah 9:6, which is clearly a messianic prophecy (Unto us a child is born…), but it also identifies this Son as “Wonderful Counselor, Mighty God, and, of course, “Prince of Peace.” This is a clear identification with the Messiah–the Prince of Peace, as God. Then there is Zechariah 11:12-13. This is also, rather obviously a messianic prophecy, given the mention of the betrayal for 30 pieces of silver. But note v. 13. “And the Lord said to me, ‘Throw it to the potter’–the handsome price at which they valued me! The “me”, here is the Lord, and is a reference to Christ..If Christ is Lord, then he is God.
Another prophecy which suggests that Messiah is God is Psalm 110:1-4. This is perhaps not a slam-dunk so much as the other two, but this clear messianic prophecy, which describes Messiah as a priest of the order of Melchizedek (Hebrews 7 tells us that this is a messianic prophecy), also describes him as sitting at the right hand of God, and as “Lord.” When Peter quotes from Psalm 110, he identifies the one at the right hand of the Father as Lord.
I am sure that there are more, but this will get you started. Like I said, there are not a lot of Old Testament statements that the Messiah is God, simply because there are only a few dozen messianic prophecies, and most of them do not specifically say that Messiah is God. But… I would conjecture that if we were to look, there are more.