One of the reasons among others why I don't take orthodox judaism seriously is its insistance that its oral tradition is equally reliable to its written tradition. You see, only with goyem, oral traditions aren't reliable, but human nature don't apply to orthodox jews (sarcasm).
Now, essentially, I highly doubt that there's any mention of an oral law in a document before the mishna (200 AD). In virtue of an oral law being oral, there is no physical proof of an oral law. This strikes me as a cause for concern because when one claims that there's an oral law, one can put anything in there and say that it's from God.
The only proofs available for the alleged oral law is "indications" from the written Torah itself, but this essentially uses a document of which there is no proof to try to establish something that there is no proof of.
Basically some of the "biggest proofs" I can remember is that some definitions of words were lost, and that one needs to know the vowels and accentuations of the Torah's words.
Is the fact that a language requires you to know the definition of words proof of a divine oral law? I think not and that such an assertion is absurd. Is the fact that Hebrew requires you to know the vowels of words proof that a divine oral law was given? Again, this is also absurd.
As far as accentuations go, Torah readings were started by Ezra and then again in 70 AD. (en.wikipedia.org/wiki/Torah_reading) Hardly proof of an oral law from Mount Saini itself.
There is one thing where instructions on how to ritually slaughter an animal are not mentioned in the Torah (as far as I've been told), and in the written Torah, God commanded the Israelites to slaughter animals in a certain way.
Ok, so if you can establish that extra information was given to Moses on Mount Saini about one particular issue (big if, the laws of slaughter could have been made up too), it does not establish that extra info was given to Moses (a whole oral law) on every particular issue. If the giving of the Torah DID indeed happen, I don't think it's unreasonable for God to have told Moses some extra stuff.
As far as the other alleged indications of an oral law goes, there is this web page www.aishdas.org/student/oral.htm where they are listed.
The problem with those is that two things, on some points it seems to assume that the existence of Case Law is equivalent to a divine oral law given to Moses. This is absurd.
The other problem is that if there is both a possible natural explanation of something (one that does not require an oral law) and a supernatural explanation of something (one that requires an oral law), the alleged indications automatically jumps the gun into the supernatural explanation despite the existence of a possible NATURAL explanation. Basically all the alleged indications do this.
In light of this blog post, read the "proofs or the oral law" page yourself, and come to your own informed conclusion.