Ok, so we agree that God gave us an oral torah, along with a written torah, but what exactly was contained in the oral torah God revealed?
I had a teacher, who currently has a position of responsibilty with one of the major Orthodox communal groups, who believed with a complete belief that every word of the oral torah was delivered at Mount Sinai, exactly as it appears today on the pages of the Mishna and the Talmud. In other words, he beleives, for example, that thousands of years before Rabbi Akiva lived, Jewish students knew that one day Rabbi Akiva, Rabbi Tarfon and the others would have their famous walk on the Temple Mount, that they knew what Rabbi Akiva would say before he actually said it.
Strange as this opinion sounds, it has support: "The Holy One blessed be He, showed Moshe the minutia of the Torah" [Meg 19b] and in Pesachim (17A) we're told that Moshe received the whole Torah, including the "comments" and even the questions an astute student will one day ask in the presence of his teacher.
According to this view, the maximalistic view, there are no new ideas, and no new rulings. It was all given to Moshe at Sinai, and the halacha Orthodox Jews follow today was contingent on nothing. Every word, every decision, and indeed our every practice was inevitable from Sinai.
A more radical view of the revelation is found in Tana debei Eliyahu Zuta where we are told that God gave man "a kab of wheat (from which to produce flour) and a bundle of flax (from which to produce cloth)." Not every law was determined beforehand. Some laws and explanations he had to determine for himself. Man isn't a passive receiver, but and active creator. God provided the principles, the raw materials, and left it to us to work out the details, to use those principles to create a system of law and life.
An intermediate position is attributed to R Yannai (P. Sanhedrin 22a) who said that the oral law "was not given as a clear cut decision;" instead "He offered 49 arguments by which a thing could be proven unclean, and 49 arguments by which a thing could be proven clean." This position accepts the maximalistic view that God literally revealed to Moshe the whole Torah, minus the final decisions. The final decisions are not inevitable, but contingent on the decisions of the scholars of each generation. According to this opinion, God does not decide. All arguments (all legitimate arguments anyway) are potentially true.
By my lights, the first position, the maximalistic position, is untenable. However, I want to keep this short, so more later.