It is. The Pali Canon was written down 2,000 years ago, and before that, for 500 years it had been transmitted strictly by a firm and consistent oral tradition.
Before it was written down, monks would wake up in the early morning and assemble in a hall to recite the discourses. They would form small groups and recite to each other to make sure that the discourse was correctly memorized. If there were any conflicts, they would consult the Elders to get the correct words and their meanings.
Venerable U Silananda: The teachings of the Buddha were handed down from teacher to pupil, first by word of mouth for over four hundred years and later published in books whenever Councils were held. Some people have doubts about the reliability of oral traditions, because they think that in such traditions, additions, omissions and distortions can be made easily. In my opinion, however, these are not possible. Those who learnt the teachings by heart and kept them in their memory were not just a few, but thousands and thousands, and they tried to keep them intact and in perfect purity. Even if they wanted to make changes in the texts, they must have the approval of all the monks who held the texts, which was impossible.
At the First Buddhist Council there were officially five hundred arahants who held the teachings in memory. In fact, there were many more such persons. If a person added or omitted something it would be easily detected at the meetings and would certainly be rejected. It would be possible only if there were only a group of a few monks holding the teachings in memory, and such fortunately, was not the case. It was to prevent additions, omissions and distortions that the was held and the succeeding Councils followed suit.
Furthermore, those who hold the teachoings in memory had too great a respect for the Buddha and his teachings to make addition and so on. Therefore, in my opinion, the Buddhist oral tradition is reliable as written records.
Venerable U Silananda held a prominent position in the Sixth Buddhist Council as the chief commpiler of the comprehensive Pali-Burmese Dictionary and as one of the final editors of the Pali Texts, the Commentaries, Sub-commentaries and other works.