Last year, A.J. Jacobs, author of The Year of Living Biblically, lived in complete observance of the literal text of the Torah. He grew a huge beard, dabbed oil on it every morning to fulfill the verse "Let oil not be lacking on your head", added safety-pin tassels to the corner of all of his clothing to fulfill the command of fringes (Numbers 15:38), and even through stones at an adulterer. Most people reading this will shake their heads in dismay and call him crazy, and yet, what is wrong with what he did? Why should keeping the literal written law be so insane? Enter the Torah she'beal pe, the Oral Torah.
The Oral Torah is not an interpretation of the Written Torah, nor a later interpolation or explanation by the rabbis. The tractate of Ethics of the Fathers in the Talmud, Pirkei Avot, begins with the words: "Moses received the Torah from Sinai and transmitted it to Joshua, from Joshue to the Elders, from the Elders to the Prophets and the Prophets transmitted it to the Men of the Great Assembly." Until the time of the Romans, the Oral Torah was passed down in an unbroken line of ordination and transmission from teacher to student. Originally, it was forbidden to write down the Oral Law for fear that the gentiles might steal it and claim it as their own, as was the case with the Written Law. During the first centuries of the common era, under intense Roman persecution, the great Rabbi Yehuda Ha-Nassi committed the Oral Law to paper because he worried that it might one day be lost. This became known as the Mishna, and centuries later in Babylon and Israel, the great rabbis known as the Tanaaim wrote an explanation on the often cryptic and concise Mishna, which is the Talmud.
The Torah writes very briefly on many mitzvot and without the Oral Law, it would be impossible to understand its many complex laws and requirements. Take the Shabbat for example. What constitutes "work"? The Torah only tells us to remember the Shabbat to keep it holy. Many sects tried to take the Torah's injunctions literally, such as "ye shall kindle no fire throughout your habitations upon the sabbath day." (Exodus 35:3), or "let no man leave his place on the seventh day" (Exodus 16:29). The heretical Karaites would celebrate the holy Shabbat by extinguishing all lights, eating cold food and sitting in the dark house all day. Could this be what the Torah had in mind as the "delight of Shabbat"? Imagine no shabbos candles, no chulent, no synagogue. Only a day of cold food in a dark home. Hardly an appealing prospect.
The Torah gives us many inferences to the Oral Law. Ecclesiastes 12:12 speaks of "making many books without end" and Genesis 26:5 records G-d as saying that Avraham kept His Torahs, in the plural, in reference to the Oral and Written Torah.
Just as a sidenote: It is interesting that there are many who mock the Oral Torah and reject it yet use Jewish symbols like talleisim, kipport, etc. to confuse Jews. And another sidenote for those who have been involved in my anti-missionary discussion: Coincidentally, the Canadian head of 'Jews for Judaism' gave a speech at my synagogue the other day so I know all of the typical missionary tricks.