Is it illegal if you don't get caught?

I had the "Is it illegal if you don't get caught" with a friend over a year ago, and our tempers flared enough that the discussion eventually had to come to an end.

I will reprise my argument here and open it up to attack and criticism, though, as always, I shall attempt to hide behind the shield of logic.  First of all, this feels like a rewording of the old adage "if a tree falls in the forest and there is no one around to hear it, does it make a sound?"  In both cases the answer is a resounding yes.

Oxford defines the law as "the system of rules which a particular country or community recognizes as regulating the actions of its members and which it may enforce by the imposition of penalties".

Furthermore, Oxford defines an illegal action as one that is "contrary to or forbidden by law".

By that definition, if an action is against the rules of conduct in the established society, it is illegal.  There is no ambiguity in either definition that could imply the requirement of an observation and charge by an  officer of the law is a necessary condition for an action to be deemed illegal. 

This in no way asserts a link between legality and morality.  The law could tell you that it is illegal to walk on the street instead of the sidewalk, but it seems a rather large stretch to claim that it is immoral.(Morality is a concept I will deal with another time anyway.)  If you perform action X, and the rules of the relevant society state that action X is prohibited, your committing the act has violated this rule and by definition broken the law.  Note again that nowhere are observation, proof, charges or a trial mentioned in the definition of "illegal" or "law", and that "illegal" is logically equivalent to "against the law."

As an extension, just because an individual is found not guilty does not mean that s/he is innocent of a crime.  This type-II bias is inherent in our legal system and the "innocent until proven guilty" structure, but does not imply that the act was not committed, or that the individual is not actually guilty of breaking the law - simply that sufficient evidence is unavailable to convince the court of their actions beyond a reasonable doubt.

Anyway, it seems there's no real philosophic debate to be had here.  If you don't like this, you can take it up with the Oxford dictionary's editors.  They may listen, consider the appalling changes that they've accepted in recent years.