I know you understand this, but people outside of DOJ often confuse the FBI’s Office of Professional Responsibly with the DOJ IG. I’m fortunate in that I’m an expert in neither aside from the mandatory death by Power Point. Basically, the OPR is the FBI’s office that maintains standards of conduct. They are the ones who come knocking when you do things like: text that hot intern describing what you want to do to her using handcuffs and a bottle of Wesson oil, use your agency credit card to buy beer, get a DUI in your “Bucar” (agency slang for vehicle), or put one through a citizen’s leg after dropping your weapon while backflipping. They generally hand out “time on the beach” (i.e. suspensions) or worse after publicly shaming the misdeeds to all of your colleagues via quarterly email.
The DOJ IG is the investigative agency for the entire DOJ. They generally handle more serious matters that often have scope beyond just one agency.
Both entities gather evidence that can be referred to a US Attorney for criminal prosecution. You can get into deep criminal kaka for lying to either as Andy McCabe is learning.