The written Torah tells us all about the oral. The written Torah will often tell us to do things, and the "how" to do it is not explained in the written Torah. It will say "do it as I told you." This alone shows that the "how" was oral.
Most Christians reject the idea of an oral Torah. Some do so based on the idea that Jesus quoted a lot from the Scriptures but never from the Talmud. Therefore, goes the thinking, Christians should reject the oral mitzvot.
This is very strange, because in the Christian bible Jesus is quoted as telling people to listen to the Pharisees, saying they sit in Moses' seat (even though there is a great deal of anti-semitism about Pharisees in the Christian bible, there are also complements from Jesus) -- and the Pharissees were the keepers of the oral mitzvot (Torah).
On top of that the oral law is actually mentioned in the Christian bible -- by Paul, not Jesus. Paul actually references the oral mitzvot in 2 Timothy 3:8 Paul names the two magicians mentioned in S'hmot / Exodus 7. Paul says: "Just as Jannes and Jambres opposed Moses" 2 Timothy 3:8 The names of the magicians are not given in the written Torah, but the names are given in the oral Torah. Paul gives those names. To the missionaries who want to totally throw out the oral mitzvot -- do you also intend to throw out the words of Paul?
Paul also claims to be something of an expert on the oral mitzvot (though it is obvious he wasn't). Paul says he: "profited in the Jews' religion above many my equals in my own nation, being more exceedingly zealous of the traditions of my fathers." Gal. 1:14 "traditions" = oral mitzvot.
Ergo Paul at least did not reject the oral Torah.
It is actually impossible to even read the written Torah without the oral Torah. . . There are no vowels in the Torah!! To even read Hebrew one must learn from the oral law.
Shabbat 31a tells the story of a non-Jew who came to the famous R' Shammai, saying to him "How many Torot (plural of Torah) have you?"
"Two,' he replied: 'the Written Torah and the Oral Torah.'
The non Jew said "I believe you with respect to the Written, but not with respect to the Oral Torah; make me a proselyte on condition that you teach me the Written Torah [only]."
R' Shammai scolded and rejected him in anger.
The gentile then went to R' Hillel who accepted the man as a student. On the first day, R' Hillel taught him the Hebrew Aleph-Bet (alphabet), beginning with Alef, beth, gimmel, dalet. . .
The next day the man returned and Hillel taught him the aleph-bet, but in reverse.
The man was confused and said 'But yesterday you taught me the opposite!"
R' Hillel explained that is the entire point -- no one can even learn the Hebrew aleph-bet without a teacher. Why rely on the teacher to correctly teach you how to read, and then not rely on the teacher with the respect of the oral law?
R' Hillel and Shammai were the last pair of זּוּגוֹת / Zugot. They lived at the time of Herod the Great -- and died shortly before the beginning of the common era (and the supposed life of Jesus). Who were the זּוּגוֹת / Zugot?
From the time of Moses there were Assemblies -- called the Elders of Israel starting in Sh'mot / Exodus chapter 3. The 70 elders (Sh'mot / Exodus 24:1,9; Bamidbar / Numbers 11:16,24) were already leading the people, even in Egypt. These men were from all the tribes and comprised both court judges and government -- later they would constitute the Great Sanhedrin. Sh'mot / Exodus 3 tells us that even Moses had to go before this governmental body and present his credentials to be accepted. "'Go, gather the Elders of Israel, and say to them, 'HaShem, the G-d of your fathers, appeared to me - the G-d of Abraham, Isaac and Jacob." Sh'mot / Exodus 3:16.
After the return from Babylonian Exile the prophets (including Ezra the Scribe) created a "Great Assembly" which comprised of 120 men rather than 71 (the 71st was initially Moses). When Shimon HaTzaddik, the last member of the Great Assembly died in 273 BCE, a period began known as the period of the Zugot, meaning "pairs." The last of the "pairs" were R' Shammai and R' Hillel from the story given above.
For almost 300 years, there were always two rabbis in charge of the Jewish courts and governmental bodies. The two were the נָשִׂיא / Nasi (the president or "prince") and the אב בית דין / Av Beit Din (the head of the Sanhedrin). These pairs are all listed in פרקי אבות / Pirei Avot / "Ethics of the Fathers." The Zugot were succeeded by the תנאים / Tannaim (teachers).
Among the first of the תנאים / Tannaim (teachers) was a name which may be familiar to Christians: the great גמליאל / Gamliel. R' Gamliel is mentioned in the Christian bible. He was the grandson of R' Hillel and became Nasi (prince) of the Sanhedrin. He died around 50 CE. R' Shimon ben Gamaliel succeeded him as Nasi (his son or son-in-law). The Romans beheaded him, and he was succeeded by his son who was known by the name of יהודה הנשיא / Y'hudah HaNasi (Judah, the Prince). Y'hudah HaNasi and his generation were the last of the תנאים / Tannaim (teachers), and it was he who finalized the Mishna (the first half of the Talmud).
Hopefully all of this detail is not boring -- I am going into detail so that you will see that none of this was "made up." These were real people (many are discussed in Roman histories as well as in Jewish history). R' Y'hudah HaNasi was an amazing man -- a learned man in Torah and in governing (he was very wealthy). He even befriended Roman leaders, including Marcus Aurelius (161-180 C.E.). This was important, because it was through his relationships that he was able to save Jewish teaching.
If you read your T'nach (bible) you know that the Jews were exiled to Babylon. While many returned to Judah (Israel) with Ezra (the prophet), many more remained in Babylon. They remained there until the 20th century when many were forced to flee as refugees -- most to Israel. By 2000 years ago there were nearly as many Jews living outside of Judah as in it. After the Temple was destroyed in 68 CE even more Jews fled, and by 135 CE the last real revolt against Rome failed and the Romans exiled most of the remaining Jews. For the next 2000 years most Jews lived spread around the world.
We went from a time of yeshivot (schools) like those of R' Hillel, Shammia and Gamliel who boasted tens of thousands of students to the very real possibility that our knowledge would be lost to diaspora (exile). Thus our leaders came to a decision -- they would have to write down the oral teachings for the very real possibility that if they did not do so they would be lost to history. This was a very difficult decision. The oral Torah was oral for a very good reason. The basic rules were firm and unchanging, but the application was meant to be adaptable to new situations (how about electricity as a new situation?).
Although R' Gamliel was a Tanna, the first Generation of the Tannaim was led by יוחנן בן זכאי / Rabban Yohanan ben Zakkai. Ben Zakkai (Jonathan, son of Zakkai) was a former student of R' Hillel and he was alive when the Temple was destroyed in 68 CE. He did a very brave (very foolish) thing to try to save Judaism. He faked his own death and had his "body" removed from Jerusalem in a coffin -- you see there were Jewish warriors who would not let anyone in or out of the city. . . His "body" was taken to the Roman leader's tent. This Roman was Vespasian and he was beseiging Jerusalem. R' ben Zakkai emerged from a coffin in front of Vespasian, telling the Roman that he had had a vision that Vespasian would soon be emperor. Vespasian laughed at him. Ben Zakkai asked a favor of the Roman, realizing that the Jews could not beat the Romans -- but Ben Zakkai hoped to save Judaism if he could not save the land. He asked Vespasian to let Ben Zakkai establish a yeshiva (school) in the town of Yavneh (near modern Rehovot). Vespasian promised that if he became emperor he would grant Ben Zakkai's request.
Vespasian became Emperor within a year, and kept his word, allowing the school to be established after the war was over. The school ben Zakkai established at Yavneh became the center of Jewish learning for centuries and replaced Jerusalem as the seat of the Sanhedrin. It was at Yavneh, nearly 100 years after Ben Zakkai, that the finalized Mishna appeared.
For the next hundred years the Tannaim in Yavneh compiled the oral Torah into the מִשְׁנָה / Mishna. The last generation of Tannaim was overseen by Y'hudah HaNasi circa 170-200 CE at which time the Mishna was finalized.
The מִשְׁנָה / Mishna is a very cryptic work explaining all the principles in sixty-two tractates. The tractates, מסכת / Mesechtot, provide the background for every subject of Halacha (Jewish law) found in the Oral Torah.
The 62 sections are divided into six orders (Sedarim):
The Chabad offers a multi-video series on the Mishna called "Tour of the Mishna."