I was replying to the above, and can you clarify that in light of your comment to me:
The OIG offices of every department are authorized separately by Congress from their parent organization and answer jointly to their respective secretary and to Congress itself, rather than to the Executive branch.
Note that the procedure is for indictments to be issued as evidence is uncovered by the IG rather than wait for the completion of a report.
From what you said above, it does not sound as though Horowitz could possibly be prevented from accessing info by a Special Counsel, as the OIG investigations don't seem to be in any way related to the SC. And the second part, I never read that the IG had the authority to indict, am I wrong there? I thought the IG recommended and then DoJ would do the indicting.
I was a little unclear. The IG does not indict. Evidence is turned over to a US Attorney where it is handled normally. The IG does not sit on evidence of wrong-doing until he issues a report, but hands it off when it’s discovered.
Sessions, Rosenstein or Mueller obstructing the OIG would be fireworks. The point of all the OIGs is that the parent agencies can’t obstruct them. That’s why OIGs are authorized separately.