This is a post for my orthodox Jewish brothers and those who may be interested in this particular debate.
First of all, let's see what was given to us:
YLT Exodus 24:12 And Jehovah saith unto Moses, 'Come up unto Me to the mount, and be there, and I give to thee the tables of stone, and the law, and the command, which I have written to direct them
Tablets of stone here are of course the 10 commandments and the law(torah) and command(mitzvah) is the Written Torah.
As you can clearly see, no other Torahs was given, and what was given, was given IN WRITING.
So even if we assume that Moses got some additional explanation from God about the Torah, it is most certainly NOT REQUIRED to interpret the Written Torah.
Written Torah is self-sufficient and can be understood, even by a child, without any additional explanations.
This is why we do not need the Oral Torah. Not to mention that it never really existed to begin with.