Two things: (1) where is Jesus mentioned in the OT and (2) how do you know he said what he said?
Now, you, as many before you have done, may quite form Isaiah and some other out-of-context verses that only imply by a stretch of one's imagination that it may be about Jesus. of course, Jewish exegesis show Christian interpretation to be a real stretch.
Then you might tell me I have to believe in order to know (!?). Sorry, that's blind faith. Obviously the fact that sciprtures have been altered, doctrinally corrupted and what not is a consideration in blind belief.
That's not arguing for the sake of argument.
I’m jumping in without knowing the context of your conversation. Just wanted to address your question about “where is Jesus mentioned in the OT.”
Take a look at Luke 24, where Jesus is walking with a couple of men and chatting about what just took place in Jerusalem. Consider version 27: “And beginning with Moses and all the Prophets, he interpreted to them in all the Scriptures the things concerning himself.” Jesus Himself recognized that Scripture — the Old Testament in this case — spoke about Jesus, and Jesus pointed those things out to the two men.
Regarding your second question: Scripture is trustworthy, and is both internally and externally substantiated. The words of Scripture confirm the veracity of other sections of Scripture, and non-biblical sources (historians, architectural evidence) also confirm the veracity of Scripture. There’s much more evidence for the existence of Jesus than any other person of antiquity, and evidence affirming that the Scriptures we have now were the ones that Jesus honored when He walked the earth.