Why is Christian theology so utterly different to Jewish theology if Christianity is Judaism+Christ?
For instance, why is there no teaching of Original Sin in Judaism but it is of central importance in Christianity? If Original Sin is based on the The Fall in Genesis then surely Jews should already have it in their doctrine?
tl;dr why is Christianity different to Judaism on OLD TESTAMENT matters
What does that mean?
Contemporary "Judaism" (the Synagogue of Satan) is based on the Talmud, not the Old Testament.
Christianity is the fulfillment of Judaism through the long promised Messiah (Jesus).
It should follow that Christianity's view of Pre-Messiah theology would be the same as Judaism. But is isn't. Why?
What do you even mean by that?
They're different because the Jews don't even follow the Old Testament. Instead, they have the Talmud, which is filled with traditions and 'corrections' on the word of God, and it's emphasized that they learn that better then the actual word of God, and the Talmud comes against much of the Old Testament. See Matthew 21:33-39
chabad.org
What don't you understand?
Christianity comes after Judaism. Christianity has Original Sin which comes from a story in Jewish scripture. The Jews don't have the concept of Original Sin in their theology despite having the scripture first.
Why do Christians differ on Old Testament theology given the fact that Jews have had the Old Testament far longer?
This picture explain well
If a Jew says a Christian doesn't understand the Torah (Old Testament) and that only the Jews understand it, how do you reply?
Have they a monopoly over the Torah ? Are jews above all other humans being and given a right that only allow them to understand it ?
More importantly; which Torah are you talking about ?
Of course Jews will claim only them can understand it, it gives them credibility.