The Oral Law contains the legal and interpretative traditions that, according to tradition itself, were given by God orally to Moses (מֹשֶׁה, Moshe) at Mount Sinai, together with the Written Law (see definition below). Therefore, according to the rabbis, the Oral Law is equal to and the only rightful interpretation of the Written Law. They were not to be written, but passed down orally through the ages.
The Oral Law contains the written rabbinic texts (like the Mishnah, the Talmud, and the Midrash) explaining the traditions of the people of Israel as well as the interpretations of biblical texts. It is called Oral Law because it was originally handed down orally, which means that young scholars (like Paul) had to learn these traditions by heart, or conveyed by word of mouth and memorized, as the Mishnah states.1
Before the destruction of the second temple, it was forbidden to write down the Oral Law (i.e. the explanation of the Torah). However, with the fall of Jerusalem in 70 A.D., the existence of Jewish culture became threatened and therefore, publication was the only way to ensure it could be preserved.2 A first redaction of the Oral Law, in the form of the Mishnah, was completed around 200 A.D. Over the next 400 years, this collection was extended through debate and discussion (Gemara) in the two centers of Jewish life, Israel and Babylonia. The Gemara with the Mishnah came to be edited together into compilations known as the Talmud.3
Who is speaking?
Besides the fact that there are quite a few contradictions between the Oral and the Written Law, another linguistic difference is quite striking. And that is the person who speaks, the subject. This is a comparison between the two:
And YHVH said to Moses: “Is YHVH’s hand shortened? Now you shall see whether my word will come true for you or not.” Num. 11:23
As you can see, like almost everywhere in the Written Torah, the Lord (YHVH) speaks directly to Moses and the people. Now the following passage is from the Mishnah, and it shows the most common wording of the Oral Torah:4
From which time are we to recite the shema in the morning? When [there is enough light so that] one can distinguish between [the] blue [strands] and [the] white [strands of the tzitzit]. Rabbi Eliezer says, Between [the colors] blue and green [and one may recite it] until sunrise. Rabbi Yehoshua says, Until the third hour [of the day], since it is the habit [of the children] of kings to rise at the end of the third hour [therefore it is within the time limit of “and speak of them … when you arise” [referring to the recital of shema (Deuteronomy 6:7) i.e., when all men, even late risers such as princes arise from their beds]. Mishnah, Excerpt from Zeraim, Berakhot 1:2
In the Oral Torah, the rabbis are the ones who speak and proclaim the different statements: They have a debate as to how the Written Torah is to be interpreted. Furthermore, there is no consensus in the Oral Torah: Different rabbis have different opinions and they rarely agree. In the Written Torah on the other hand, there are no contradictions. YHVH never contradicts himself.
The Karaites reject the Oral Law and rely only on the Tanakh (Old Testament) as acceptable scripture. Some Karaites go as far as only accepting the Peshat meaning (plain meaning, see chapter on PaRDeS) of the text.
1 Mishna, Seder Nezikin, Avot 1:1. The manner of teaching and memorization is described in Seder Mo’ed, Eruvin 54b
2 Tosefta on Seder Nezikin, Eduyot 1:1 “When the Sages went to Yavneh they said: The time will come that a man will seek a matter in the Torah but will not find it. He will seek a matter from the Scribes but will not find it…They said: Let us begin [to record] with Hillel and Shammai.”
3 Article on Wikipedia
4 Translation by emishnah.com