We always say "Everything was given to Moshe at Har Sinai." But then there's also that story in the gemara about Rabbi Akiva and Moshe. Moshe is being shown Rabbi Akiva teaching Torah to his students in the second century CE. Moshe watched and was startled; this was not the Torah he gave the Israelites. Then he heard students ask: “’Rabbi, what is your source?’ He answered them: ‘halakhah l’Moshe Misinai'” Moshe was relieved.” While realizing that Rabbi Akiva’s teaching was not what he taught, he was consoled, for Rabbi Akiva was saying these laws are derived in some way from Moshe and they are as significant as if Moshe obtained them at Sinai. So when was the Oral Law really given?
Rav Naphtali Zvi Yehudah Berlin explains in Perek 4 that Devarim was the original Oral Law. Moshe tell Israel to listen to the "Chukim" and "Mispatim." We have already many Mitzvot. What are these new ones? The Netziv explains that "Chukim" are the 13 principles on how to interpret the Torah and the "mishpatim" are the new laws derived from these interpretations. According to the Netziv the way the Oral Law came about was through these chapters in Deuteronomy. The Netziv says that in these chapters, Moshe shows Israel these new principles and the laws that Moshe himself derived from them. This taught Israel how to do the same in future generations.
Therefore, both sides are right. Moshe never taught the specifics of the Oral Law and all of its new halachot, but he did teach how to achieve the new laws through chukim and mishpatim.
Thank you netziv
