The Old Testament was actually compiled after the New Testament, as a Jewish response to the Christian New Testament. Many books, and portions of books, were cut from the Jewish scriptures, some after hundreds of years in use, in an attempt to "purify" the scriptures from perceived corruption and watering-down of the original messages.
During the Reformation, Protestants similarly purged their Old Testament books of the later additions. Catholic Bibles retain some of the books (called the Apocrypha), but cut out others; Orthodox Bibles retain even more.
Another answer
Jewish tradition states (Talmud, Bava Batra 14b-15a) that each prophet wrote the book that bears his name. The Tanakh (Jewish Bible) was sealed by the Sanhedrin (council of sages) called The Men of the Great Assembly, whose members included the last of the prophets. This was around 2350 years ago.
The Old Testament.
Abraham is first mentioned in the Old Testament.
Jonah is from the Old Testament.
The book of Job can be found in the Old Testament.
Jesus was in the new testament and not the old testament.
The book of Psalms is found in the Old Testament.
There are 39 books in the Old Testament and 27 books in the New Testament (a total of 66 books in the Bible). There are 23,145 verses (KJV) in the Old Testament and 7,957 verses (KJV) in the New Testament. If all verses were equal, that would put the Old Testament at 74.4% and the New Testament at 25.6% of the entire Bible.
Psalms are in the old testament.
Old testament
The Old Testament is the basis of Christianity.
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The old testament was addressed to the Jews.