The Old Testament (OT) books of the Bible are translations of the Hebrew Bible that are still used today by Jews in their worship services. These books were written by many different authors over a period of hundreds of years.

The Old Testament developed gradually and the books were first collected in small groups. Before beginning the process of collecting the books and arranging them in a particular order, individual manuscripts were recopied and circulated among the congregations. The oldest writings in the OT can probably be dated back to the time of Moses or even earlier (around 1300 BC), while other books (e.g. the book of Daniel) were probably written only in the 2nd century BC. That means the books now collected in the OT, were written over a period of about 1000 years or more!

While the writing of the Hebrew texts was going on, the process of collecting and editing the books was also going on. One important collection of books is called the Tora (“Torah”) or in Greek  Pentateukh (“five scrolls”). Another collection of books is called "Prophets", which also includes some books that in the Christian tradition belong to the "historical books" group. The last collection of books is simply called “Writings”, which consists of various poems, words of wisdom, and some books that in the Christian tradition are classified as prophetic or historical. Because different religious traditions compiled these books differently as well.

It is not known exactly how or when the books that are now in the OT were first selected and approved as part of the Hebrew Bible. However, it is almost certain that the five books included in the Torah were accepted as authoritative from the beginning of their writing. There is evidence that the official list of authoritative books was not made until after 100 AD. It is only the books on this latter list that are considered holy books by the Jews.

The three main sections of the Hebrew Bible, namely the Torah, the Prophets, and the Writings, when compared to the Christian Bible, we will see that all these books are present in the OT of the Christian Bible, albeit with different groupings and placed in a slightly different order. The Hebrew word for their Bible is Tanak (sometimes spelt Tanakh). It's an acronym formed from the first letters of the three main sections: Tora, Nebi’im, and Ketubim.

Certain Christian traditions also incorporate other books into their OT. These books are known as the Apocryphal or Deuterocanonical books. The term "apocryphal" comes from the Greek word meaning "hidden", a name that signifies that the books have been "set aside" or given secondary status. The term "deuterocanonical" is preferred by Catholics to refer to many of these books. It refers to books that entered the canon from a later (secondary) age, to distinguish them from the Hebrew Bible discussed above. In the 1600s, some Protestant Christians began to use only the list of OT books as used by the Jews, while the Roman Catholic and Eastern Orthodox churches continued to use some or all of the Apocryphal/Deuterocanonical books.

Source: Bible Study Edition