Due to the Inspector General Empowerment Act of 2016, the OIG has access to all of the information that the target agency possesses. This not only includes their internal documentation and data, but also that which the agency externally collected and documented.
The FBI/DoJ documents EVERYTHING, even little memo’s that they write to themselves (Comey). The bulk of Michael Horowitz mountain of evidence comes directly from the DoJ/FBI. The IG Empowerment Act of 2016 legally compels them to largely tell upon themselves. By having access to the information that the DoJ/FBI has, the investigatory power of the DoJ OIG is increased exponentially. The OIG pours over the agencies own data and documentation and creates investigatory leads based upon them. That is where the agents come in. Note: Robert Storch was nominated to the newly formed position of OIG of the NSA and was confirmed on December 21st, 2017.
It has been reported that the DoJ OIG has a staff of anywhere from 400-500 and a budget of $100 million. Here are the actual numbers pulled from their 2018 budget. 
The Investigative Division investigates the investigators. One way to view them is as the internal affairs of a police department. They are purists, are squeaky clean, and absolutely despise dirty cops/agents.
In addition to the Investigative Division, Mr. Horowitz’s other divisions “conduct special reviews requested by the Inspector General or senior department management that arise suddenly and need immediate attention.” For example the audit division. The audit division is a crack team of auditors that specialize in pouring over mountains of data and data systems. Their primary function is financial forensics. They are available in a supporting role.
The Oversight and Review Division functions as a divisional coordinator/review board for “investigations of a sensitive allegations.”
In addition to the internal resources available to Michael Horowitz, we know from past behavior that he will expertly use the House and Senate Judiciary Committees to achieve his aims. READ THIS 👇👇👇 
In my research into Horowitz I was fascinated by the way he forged an alliance with Congress and how they worked together to defeat the Obama Administration. It was a thing of beauty and precision. You can count on me to cover this aspect in detail. His war with the Obama Administration is a great case study in how he operates. The tactics will be the same with two highly significant differences. Public disclosure and a new administration that is not adversarial. Horowitz in 2017 took his organization to a new level via public disclosure. He no longer wants his findings hidden from us by the media. In May he created the twitter account @OversightGov. On October 1st his website oversight.gov went live.
Despite what the conspiracy theorist profess there is absolutely ZERO EVIDENCE that the DoJ/FBI will not comply this time around. It is all conjecture. People are making the foolish mistake of viewing the current leadership of the DoJ the same way as Lynch/Holder/Comey etc. The black hats are no longer in charge of the DoJ/FBI. These guys are.
The top law enforcement official in the land is President Donald J. Trump. He sets the tone for the DoJ just like Obama set the tone for Lynch/Holder.
President Donald J. Trump on making the FBI great again: “It’s a shame what’s happened with the FBI. We’re going to rebuild the FBI, it’ll be bigger and better than ever.” The Trump Administration and R’s in Congress will take full advantage of the undisputed neutrality of the Office of Inspectors General. It will allow them to use his findings as a safe launching point with which to take actions and investigate further. Imagine the collective hysteria and opposition if Sessions, Trump, or Congress called for a special counsel into the 2016 Election or attempted to re-open the Hillary Clinton investigation without an independent and politically neutral IG Report. I think the Trump Admin might have done it anyway but seeing as they were notified in January 2017 of the IG’s activities they did not have to. This is the beauty of the IG Report. It functions as a shield from which to launch the attack.