Oracles do not exist. If one did exist, then you would replace them with a subroutine with computational requirements and you would no longer need an Oracle.  Thus, Oracles do not exist almost by definition.  I don't understand how one can make an mathematical argument based on something that does not exist (excluding unbounded memory machines, of course).  Have a problem, ask the magic Oracle, problem solved $O(1)$ time.  Of course you can't say anything about what is happening in the oracle, because its magic. From this point of view, Turing Machines (bizarre fetish formalism if you ask me) with Oracles do not exist.  Also, that X historic proofs rely on oracles makes oracle arguments valid is a circular argument.
How is it valid to use oracles in mathematical arguments?
complexity theory;turing machines;proof techniques;p vs np;oracle machines