I'm not aware of any summary reports on the effectiveness of the OIG, but I've read that its structured to try to minimize the most obvious types of corruption (for instance, by also reporting to Congress and not just to the agency head). I'm sure there's plenty of mileage variation depending on the circumstances and political environment.

This page suggests that the office does investigate misconduct claims and issue reports periodically: https://oig.justice.gov/reports/type/investigation