According to what I saw the other day, Obama put the OIGs under the SES - Swamp Critters Extraordinaire
They are mutually exclusive. Both IG and SES are creations of Congress. IG are for purposes of oversight of the executive. SES are for purposes of continuity between administrations.
SES specifically excludes positions requiring appointment by the President with Senate confirmation. See 5 USC 3132(a)(2). IG's are nominated and go through Senate confirmation before appointment. See 5 App. USC 3