"Officially" they were all written by Moses. Only the new testament has different authors.
Well that's not even what Christian scholars say about the OT.
Traditionally, both Jews and Christians believe Moses is the author and compiler of Genesis and each of the first five books of the Bible also called the Pentateuch:
These are the five Books of the Law: Genesis, meaning "beginning" since it recounts the beginning of God's creation; Exodus, which means "exit" or "departure," referring to the journey of the Hebrews from out of slavery in Egypt; Leviticus, a book detailing worship as led by the priests ordained from the tribe of Levi; Numbers, whose title is derived from the book's opening account of the census or numbering of the people of Israel; Deuteronomy, meaning "second law" since it gives a detailed listing of the additional laws given by God through Moses.
This second group of the Old Testament is known as the historical books. Job - Unknown Psalms - Various: David is credited for 73 of the Psalms, Asaph is credited for 12, Sons of Korah for 11, Solomon for 2, Moses, Ethan, and Heman for 1 each, and 50 are unknown. Proverbs - Various: Solomon is credited for 29 of the Proverbs, while Agur and Lemuel for 1 each. Ecclesiastes - Solomon Song of Solomon - Solomon This third group of the Old Testament is the books of Wisdom, proclaiming that happiness is possible only through faith in and obedience to the one true God. Isaiah - Isaiah Jeremiah - Jeremiah Lamentations - Jeremiah Ezekiel - Ezekiel Daniel - Daniel Hosea - Hosea Joel - Joel Amos - Amos Obadiah - Obadiah Jonah - Jonah Micah - Micah Nahum - Nahum Habakkuk - Habakkuk Zephaniah - Zephaniah Haggai - Haggai Zechariah - Zechariah Malachi - Malachi