alt Hacker News>OIGs are structured to be independent specifically to avoid the "investigating yourself" problem.
You ever worked in an organization that had an OIG and met the folks that work there? They wear the same uniform you do, and their behavior betrays that fact.
I do have some experience, and while I certainly won't argue they're perfect, my observation has been that they're more than capable of delivering conclusions that reflect negatively on an organization. Dismissing their conclusions out-of-hand in favor of an alternative theory with no evidence feels like just believing what you want to believe.