As our teacher, Moses (Moshe) received each of the commandments (Mitzvos) and their oral commentary from G-d, tremendous care was taken to ensure that even the smallest detail was transmitted correctly.
First, Moshe would repeat G-d’s words to his brother Aharon. Then he would repeat them again, to Aharon’s two sons. Following that second repetition, the elders would enter and Moshe would teach the new Halacha again. Moshe would teach the subject a fourth time to the whole nation. They would then break up and hear it from Aharon, his sons and the elders. No one was “finished” with that lesson until he had gone over it at least four times. For the forty years in the desert, that kind of attention to detail was the order of each day. By the time the Jews entered Israel, they were a nation which was expert in the most complicated of the laws.
But what happened to the next generation? And the one after that? How do we know that the oral law we have today (i.e. the Talmud and its many commentaries) is that same one that was received by Moshe? There are many answers to that question, but at the essence of each of them is the quality of the transmission.
Each generation had thousands of scholars dedicated to learning and teaching the oral law, but in each generation there was one man (or sometimes two) who was responsible for everything. We know the names of each of those men (see the introduction of Maimonides the Rambam to his work “Mishna Torah”) and a great deal about their personalities and careers as well. The eighteen hundred years between the giving of the Torah (2448 1312 BCE) and the compilation and writing of the Talmud aren’t just a blur of a few names and myths…. They are a period that is open to us and about which we have a complete historical record.
One other point: When Rabbi Yehuda Hanasi decided that the time was right to write down the Mishna, he gathered together the greatest living sages from across the known world. Many of these men had never seen each other and might have lived in communities separated one from the other for centuries. Yet, when they had gathered all the versions of the oral Torah, there were arguments about some relatively minor details of some Halachos (laws). Nevertheless, they found that there was complete agreement over the Mitzvos (commandments they all agreed on each of the 613), and over nearly every point of Jewish life and law. And this was after some 1500 years of life under oral law!