In the Torah – Devarim ch.28, we see a warning , to avoid all the plagues and curses in the Torah –
58 If thou wilt not
observe to do all the words of this law that are written in this book, that
thou mayest fear this glorious and awful Name, the LORD thy God;
This is referring to the words written in the Torah.
The problem this raises for proponents of the alleged oral Law is that it makes no reference to the words in the Talmud or mishnah. Thus, for example, the claimed festival of Nisuch Hamayim – Water Libation, which the Pharisees imposed during the Sukkot festival, is not mentioned in the Torah. As is the case with all other rabbinic and supposed non-written Laws.
Had there been a contemporaneous oral law, at the time of the giving of the Torah, the Torah would also be warning us to observe those extra laws as well, which it clearly doesn’t. This omission is very significant, since it exposes the absence of the oral law from Sinai.