I read Oral laws were forbidden to be written down by MOses and then due to a threat people started to write it down , Hence curious to know which is the first manuscript on Oral law , most probably it seems to be written around the time of the pagan roman empire which was considered as threat
What are the earliest sources of Oral Law irrespective of Jewish denomination like Kariate Samaritan , acceptance etc
I'm not sure if this is precisely what you mean, but the oldest rabbinic text (so old in fact that it's perhaps better described as proto-rabbinic) is Megillat Taanit. It comprises a list of dates on which it is forbidden to fast, or on which it is forbidden to recite a eulogy. There is an accompanying text that scholars term the scholion, which serves as a commentary on Megillat Taanit, describing what happened on each of those days. The text (including the scholion) is so old as to be referred to in the Mishna (eg: Taanit 2:8), and is our earliest source for the story of the miracle on Hanukkah.
If you are asking for information on the oldest text (ie: not which text was composed before all of the others, but which textual remains are, physically, of the greatest antiquity), then the answer would be the material found in the Dura Europos synagogue in Eastern Syria, dating from 244-256 CE. To quote from Stephen Fine, "a parchment fragment discovered [in Dura Europos] in 1932 is highly reminiscent of rabbinic prayer texts; inscriptions closely parallel Targumic paraphrases of Scripture in Aramaic; and the wall paintings suggest important parallels with midrashic sources." [source].
While not the oldest, strictly speaking, a very old (and very interesting) example of this phenomenon can be found in the synagogue in Rechov. Dating to the 5th-6th centuries, there is an inscription there that, in the words of Stephen Fine, is the earliest preserved halakhic text yet discovered. The text of the inscription directly parallels passages in Tosefta Shevi'it 4:8-11, Sifre Deuteronomy 51 and Jerusalem Talmud Demai 2:1, 22c-d and Shevi'it 6:1, 36c. According to Prof. Saul Lieberman, it was probably the transcription of a letter sent from a bet din to the synagogue in Rechov, treating of the laws of tithes in the sabbatical year. The full text can be read in English at the same site to which I linked above, which was Stephen Fine's article, "The Rehov Synagogue Inscriptions: The Earliest Preserved Text of the Talmudic Literature".
The idea that it was forbidden to write down the Oral law is a bit of an over simplification.
Students and scribes always wrote down their methods and notes on various issues. What was forbidden was the collection of these notes into a system of law. Each person in their own community was meant to seek out the wise people of where they lived and the judges and they were meant to rule based on oral tradition passed on to them.
After the destruction of the second temple, there was fear of people being unable to find for themselves a scribe or wise person to learn from. Thus we have the creation of the Mishna.
However, Bereitot and Mishnayot were written down and kept individually in the homes of various scribes long before the Mishna was actually compiled and codified. These smaller notes are known to exist all the way back to the era of the Greek Conquest of Israel.
Another way to look at this question is that any nuance of halacha in Tanach would have been oral law until it was written within. So one can argue that the earliest writing down of any oral law would have been when the first book outside of the Humash was transcribed to be made part of the Tanach. In the eyes of the Halacha however this would not count, since those books are not systemic books of law, rather they just happen to contain some law within them in passing.
Both Samaritan and Kaarite texts where not written until well after the Talmud was published, and there are no existing collections of oral laws predating the Mishna.
