The question of oversight has become of increased interest to me as of late, due to my research into the NATO effort in Afghanistan. A key contrast between the US and Canada is that the legislature in the former has far greater ability to investigate the secret stuff (due to security clearances, staffers, etc) than the latter. And now both have on-going investigations.
In the US, the concern du jour is of the role of private security contractors, especially the notorious Blackwater Company, in CIA operations. In Canada (as I will post in an hour or two), the question is of who knew what about the condition of Afghans who had been picked up by the Canadian Forces but then turned over to the Afghan authorities. There are reports that some of these folks were beaten, which resonates deeply in Canada (again more later).
Are these examples of failed oversight? Or successful oversight? Not clear in either case, but oversight, as far as I understand it, in these cases is supposed to detect wrong-doing and the threat of such detection is supposed to deter wrong-doing in the first place. BUT oversight in these forms (fire alarms to be pulled when there is a suspicion of a problem) does not really stop something from happening. There is no commissar on location that can stop the folks from acting or make them act.
I do think the reliance on private contractors is a key means by which actors within the US try to evade oversight. PMCs have much less accountability to Congress, so there is a temptation to rely on them rather than American troops. Combining CIA operations, which are already difficult to oversee, with PMC's is, in the immortal words of Jimmy James (a character on Newsradio, a sitcom): smothering it with secret sauce.