And what does he have to say about it?
Based on the OT only you get the Jewish view of God, which is mutually exclusive with the Christian one, ergo, they're different gods.
Marcion said the God of the Old Testament was different from the God of the New. He wanted to discard the Old Testament and only wanted to recognize the writings of Paul, and a few others, as being authentic.
After some discussion, the Church declared him a heretic.
You do have a good question though, and I had to think a while on how to answer it.
Christianity is not Judaism. This is true and Jesus is quoted as saying as much. He said, to paraphrase a parable, that you do not put new wine in old wine skins. In other words, what he was teaching was different from what Judaism was in the first century.
However, we believe he is the Son of God because he said so and then raised from the dead. He was also understood to say that he was the same as the God of the Old Testament and that the Jews had misunderstood and perverted his message.
This is standard Christian belief, so if this is true, what can we deduce from what Mohammed said?
He contradicted most of Jesus' message and said that salvation was based solely on how well you performed your duties as commanded by God; not unlike Judaism. He also denied that Jesus was who he said he was or that he was even crucified.
Both of these two beliefs cannot be true because they specifically contradict each other on Jesus, the nature of God, and the message of salvation.
We worship the God who gave us our message and you worship the spirit who gave Mohammed his message. Either they are different or God is a liar or one of two men made something up.