It is not wrong to say what you genuinely believe regarding the Old Testament, providing it does not conflict with the teachings of the Catholic Church. An element of the question is:
Jesus is God, the Father is God, and the Holy spirit is God.
With this is mind, is it wrong to say Jesus is the God of the Old Testament?
According to Christian belief, Jesus, the Father and the Holy Spirit are one, which means that at one level, all three are the God of the Old Testament. However, we are also shown that Jesus can speak and act independently of the Father, even praying to the Father (eg Mark 14:35). Likewise, the Father acts independently of the Son (and the Holy Spirit likewise).
There is nothing in either the Old Testament or the New Testament that demonstrates that any reference to God in the Old Testament ought to be read as referring to Jesus rather than to the Father. Some believe that when God appears in the likeness of a man (eg Genesis 3:8ff), that this must be Jesus, since the Bible says elsewhere that no one can look at God's face and live. However, these differences are readily explained by differences of authorship, with Genesis chapter 3 widely attributed to the 'Yahwist', a source for whom God was anthropomorphic. The Catholic Church (and most other Christian denominations) teaches that Jesus is only prophesied or foreshadowed in the Old Testament, and that it is the Father about whom we read in the first part of the Christian Bible.
Returning to the question:
There was a group called the Marcionites that argued that there are 2 Gods
because the God of the OT and the NT are just too different...
Marcion (85-160 CE) not only knew nothing of the Holy Trinity, he believed that God the Father could not have been the Creator God of the Old Testament, whom he called the Demiurge. The Catholic Church excommunicated Marcion because of his teachings, so clearly they should be regarded by Catholics as entirely wrong.