Secondly, there couldn’t have been an Oral Law because in the time of King Josiah, they had lost the Book of the Law and it appears that they didn’t even know what Passover was or certainly how to celebrate it! The Temple was in ruins and the King ordered its restoration. In the midst of this great undertaking the Torah was recovered.
Hilkiah the high priest said to Shaphan the secretary, “I have found the Book of the Law in the temple of the Lord.” (1 Kings 22:8)
The king called all the people together and they read the Book of the Covenant. Together, they renewed the covenant with the Lord. King Josiah ordered that the Passover be celebrated.
The king gave this order to all the people: “Celebrate the Passover to the Lord your God, as it is written in this Book of the Covenant.” Neither in the days of the judges who led Israel nor in the days of the kings of Israel and the kings of Judah had any such Passover been observed. But in the eighteenth year of King Josiah, this Passover was celebrated to the Lord in Jerusalem. (2 Kings 22:21-23)
To summarize, the Torah had been lost as the Temple was in ruins. The king of Israel and the priests did not even know what Passover was—or at least, the details of proper Passover observance. Since the Mishna (the Oral Law in writing, as part of the Talmud) speaks of the Passover at length—in fact it has an entire tractate (major section) called Pesachim (Passovers) that teaches in incredible detail how to correctly celebrate Passover—it had to have been created after the time of Josiah.
In addition, had there been an Oral Law passed down from Moses it was certainly forgotten. And unlike like a Written Torah, that could be found in the ruins of the Temple, it would be impossible to recover an Oral Torah.
Third, we find an interesting passage in the Torah that refutes the idea of a non-written Torah.
When Moses went and told the people all the LORD’s words and laws, they responded with one voice, “Everything the LORD has said we will do.” Moses then wrote down everything the LORD had said. (Exodus 24:3-4a)
Could it be any clearer? God shared all His laws with Moses and then Moses wrote down everything. In the Hebrew it says Kol Div’re Adonia—all the words of the Lord. There was no secret Oral Tradition; all was written. (Here are a few more passages you can reference: Deuteronomy 30:10, 31:9, 24, 26, and Joshua 1:8).
And fourth, one primary reason the Word of God needed to be put down in words was to protect Israel from deception. An Oral Torah would have led to all kinds of duplicity and many would have changed it for their own purposes. Keep in mind, the Children of Israel, my ancestors, went through many periods where they forsook the Lord. Not only would an Oral Law have been abused by leaders during such a time—it would have been eventually ignored and utterly forgotten.