You may be one of those who know this heretical teaching among the rabbinists—may they be anathema—that God gave Moses two sets of laws: one being the written law, and the other being the oral law. A pair of apologists on the Internet—names of Eitan Bar and Moti Vaknin, if memory serves … heck, let me find the link—have dealt with this issue, already, from a well-informed and sufficiently-compassionate angle. There is a video, which is actually what I watched.
Nevertheless, as these things go, since it is a blind and accursed heresy, every other page in the Bible screams evidence against it. —But my favourite is Exodus 17:14, where “The Lord said to Moses ‘Write this for a memorial in a book, and speak it in the ears of Joshua …’” That is, though you start an oral tradition by Divine command, yet also record it in a book for a memorial; moreover, the information is that the Lord will “utterly blot out the memory of Amalek from under Heaven”, and wage war with Amalek for all generations.
Such a truth, which shall be evidenced and be fulfilled from generation to generation would very well survive not being written down. It is not nearly as intricate or “random” as the precepts of the Lord that Moses gave the Israelites. And yet, “write it down in a book.” If anything could support the “oral law” heresy, it would be this verse; and yet the verse denounces it roundly, from the mouth of God Himself.