Overview: Despite the evidence suggesting there is an Oral Law that accompanied the Written Torah, the lack of explicit mention of the Oral Law within the Written Torah begs explanation.
While this question wouldn’t invalidate the Oral Law if unanswered, it does feed major skepticism—if unanswered. If the Oral Law truly exists, shouldn’t we find explicit references to it within the Written Torah or Tanach?
So, one point needy of mention is the fact that there is no place in the Bible that there was a necessity for the Oral Law to be mentioned, rather there are perhaps places where it could have been mentioned yet it wasn’t.
[Also, as explained earlier, the only (even possible) alternative for the Oral Law, namely, the theory that the High Court decides the details of the commandments that weren’t explained, as well doesn’t have any scriptural reference or even implicit reference (unlike the Oral Law which does have implicit references, as elaborated upon earlier).]
Another crucial point is that we clearly see how G-d does not prove His existence and His Torah in the most absolute way. Thus we must not expect Him to prove the existence of the Oral Law either beyond the shadow of a doubt.
To address this issue, we will use two different approaches based on two diverging opinions found in the Talmud.[i] The first view believes that the original size of the Oral Law, i.e. the way it was at Sinai, surpassed the size of the Written Law. According to the second opinion, however, the Written Law was larger than the Oral Law. Although nowadays the Oral Law is significantly larger than the Written Law, according to the second opinion this wasn’t always this way. The Oral Law we have today can be split into three categories. (1) Laws directly from Moses at Sinai. (2) Rabbinic interpretation of Torah using the derivation tools. (3) Rabbinic enactments. All these three categories were discussed earlier. So according to the second opinion, the first category is rather small and the vast bulk of today’s Oral Law consists of the second and third categories.
This fundamental argument will leave us with two approaches in answering the question of why there’s no explicit reference to the Oral Law within the Written Law. The first approach is based on the first opinion that the Oral Law exceeded the Written Law.
By this approach we can compare the Written Law to a school textbook. The textbook distributed by the teacher is incomplete, and an oral explanation by the teacher is required. In the actual textbook, though, no indication of the teacher’s explanation is (necessarily) written clearly; it is rather clear from the limited understanding of the material that the explanation exits. There is no need for a clear indication, for the textbook and its explanation come simultaneously. The same applies to the Written Torah which is accompanied alongside its oral explanation (both by Sinai and from generation to generation), which it so clearly needs.
It is clear from the Torah that the written document of the Law was preceded by the oral knowledge of the Law. That means to say, that the Torah was only written (or more likely compiled of the many existing separate documents that Moses wrote throughout the years) at the end of the 40 years of the sojourn in the wilderness (See Deuteronomy 31:9). Until then, the Jews kept the Law based on the oral teachings of Moses and the judges (See Exodus 18). It was only after this Oral Torah that the Written Torah was documented. It is now that the question arises whether or not this Oral Law—that definitely existed before the Written Torah was documented—contained only the laws written in the Torah, or that it contained more details that were not documented but were intended to be orally transmitted.
This idea of something being oral, apparently is not something new in the Torah. In the Torah, G-d fails to describe Himself (i.e. His characteristics) and why He is different than the other gods of wood and stone that the Torah frequently criticizes. As a matter of fact, there’s no clear place where G-d describes Himself as infinite, but rather simply prohibits serving other G-ds (although it is clear that He is infinite after analyzing the verses[ii]). We thus see that G-d didn’t only intend for the scriptures to serve as testimony for His existence, for if so, He would have written it clearly. Rather it is the oral testimony from generation to generation that provides clear testimony of such a fundamental concept, namely, the characteristic of our G-d.
It is in this theme that Deuteronomy Ch. 4 elaborates how the generation of the Exodus saw that G-d is infinity and that this message should be passed down to their children orally (note that the verse does not say to show your children the scriptures but emphasizes to teach them orally). Similarly goes to some other fundamental concepts which although alluded to (and even can be proven from the Written Torah), they were seemingly hidden from the texts. More particularly this includes the World to Come, the spiritual soul, and the Messianic Era. The details and clear explanations of these concepts are embodied in our Oral Tradition which the Written Torah relied on…
Here’s another possibility as to why the Written Torah hasn’t clearly referenced the existence of the Oral Law:
We explained earlier why the actual laws and explanations of the Oral Law were not written down by Moses. The main answer we gave was that the Gentiles shouldn’t be able to take the Oral Law from us just as they have done to our written Torah. If our Oral Laws would have also been documented, the Gentiles would have possessed it as well. How then would we as Jews feel different than the Gentiles? How would we feel special and chosen? Which Word of G-d would we then exclusively possess?
The very same dilemma would apply if the Oral Law was explicitly mentioned in the Bible. If they would clearly see that there had to have been an oral explanation (because the Torah would have very clearly referenced it), then they would steal those laws too (perhaps Ptolemy would force those very same rabbis to write down for him the Oral Law). Now that there is no explicit reference and the Gentiles didn’t and don’t intend to keep the laws—and therefore don’t put much heed to the fact that the laws aren’t properly described—they’re comfortable enough with the Written Torah, and in fact, deny the Oral Law…
The second approach to the question of why there’s no reference to the Oral Law within the Written Law is based on the second opinion brought earlier. According to the second opinion, the Oral Law was small at first. We can even suggest that it was so small that only few dozen laws and principles are actually from Sinai. We addressed this briefly in chapter “Is There an Oral Law, pt. 2”.
Based on this assumption, it is no amusement why the Written Law doesn’t mention the Oral Law—it was simply insignificant at the time. Nowadays, the Oral Law plays a significant role in Jewish life due to its immense size; but back then it was a fraction the size of what it is now. Thus no mention of it ever came up in the Written Torah. This is even the more so because these original oral laws were only details to laws already written in the Written Torah. The critics who complain that the Oral Law isn’t mentioned in the Written Torah, generally mistaken the Talmud to be all from Sinai. But that us far from the case.
 Tradition has it that Ptolemy, king of Egypt, forced the rabbis to translate the Written Torah into Greek. It was from this that the Christians and ultimately Muslims—as well as every other religion that believes in the Bible—stole the Bible from the Jewish People.
[i] Gittin 60b.
[ii] Genesis 1:1, Deuteronomy 4:35, 39 and many more in the Prophets and Writings.