Which agency is not overseen by the Office of Inspector General (OIG)?

The Drug Enforcement Administration (DEA) is not overseen by the Office of Inspector General (OIG) because the DEA operates under the Department of Justice (DOJ), not the Department of Health and Human Services (HHS). The OIG specifically focuses on issues related to the HHS and its agencies, which include organizations like the National Institutes of Health, Health Resources and Services Administration, and the Centers for Disease Control and Prevention. The OIG is tasked with promoting efficiency and preventing fraud, waste, and abuse in HHS programs, and its jurisdiction does not extend to the DEA's regulatory or enforcement activities related to controlled substances. Thus, among the given options, the DEA stands out as the agency that falls outside the purview of the OIG.