(Bloomberg Opinion) -- The Office of Legal Counsel can be seen as the Navy Seals of the U.S. Department of Justice. It consists of a relatively small, and quite powerful, group of lawyers who provide legal advice to the president and the Cabinet departments, often on the very hardest questions.If the State Department and the Department of Homeland Security disagree about a legal issue, OLC, as it is called, might well be asked to settle their dispute.If the question is whether Congress can require the president of the United States to hand over his tax returns, or whether the president can fire members of the Federal Reserve Board, or whether executive privilege applies to conversations not involving the president personally, or whether the president is immune from criminal prosecution – well, there is a good chance that OLC will have the final word, at least within the executive branch.This helps explain why Robert Mueller deferred to a crucial judgment of the OLC, to the effect that the president is immune from criminal prosecution as a matter of constitutional law. The special counsel was criticized for following the office’s opinion, but he was right to do so. Robert Mueller is a straight shooter.You might well ask: Does OLC operate as a court? Is it supposed to provided neutral legal advice? Or is it in the president’s pocket? Those are excellent questions. In different administrations, they receive different answers.OLC is headed by an assistant attorney general, nominated by the president and confirmed by the U.S. Senate. President Donald Trump’s OLC head is Steven A. Engel, who worked in the office under President George W. Bush. It is not unreasonable to suppose that Engel, like his Democratic and Republican predecessors, is inclined to be sympathetic not only to his particular boss, but also to the constitutional prerogatives of the president.So there’s a hard balance to be struck.(3) By tradition, OLC has a degree of independence, and for that reason, it tends to receive bipartisan respect. By contrast, the White House counsel sits close to the president and is closely attentive to his wishes. OLC’s staff, which consists of civil servants, prides itself on its devotion to the Constitution and to the law, as such -- to its ability to say a firm “no” to the president, whether he’s a Republican or a Democrat.At the same time, OLC is part of the executive branch, and it has a long history of resolving tough constitutional questions in the president’s favor – and occasionally of stretching the law. It is also aware that if it says “no” too often, or on the wrong occasions, the attorney general is unlikely to be pleased, and the president might get irritated or worse. Under both Democratic and Republican presidents, the White House has occasionally tried to bypass or avoid OLC, or to diminish its authority, on the ground that the office might give it an answer that it won’t like.Turn to the Mueller report in this light. In a key passage, Mueller accepted OLC’s controversial conclusion that a sitting president may not be subject to criminal indictment. He did so without analyzing the issue on his own, or explaining why and whether he thinks that OLC got it right.Instead he made an institutional point. His office was part of the Justice Department, and for that reason, Mueller followed OLC, whether or not he agreed with it: “Given the role of the Special Counsel as an attorney in the Department of Justice and the framework of the Special Counsel regulations, this Office accepted OLC's legal conclusion for the purpose of exercising prosecutorial jurisdiction.”Some people did not love that sentence. They think that OLC’s conclusion was wrong – that no one is above the law, which means that if the president has committed a crime, he can be indicted like anyone else. They also think that Mueller should have made up his own mind.But Mueller made the right call. Whether or not the Office of Legal Counsel is correct, a special counsel within the Justice Department is bound by its legal conclusions; it has no power to reject them.There is a broader point here. With each passing week, the Mueller report looks increasingly impressive, because it is scrupulously fair, and also because it is by the book. That means it is bound to disappoint and even enrage the president’s sharpest critics and most enthusiastic defenders. Robert Mueller is a straight shooter.(1) I served as an attorney-adviser in OLC under Presidents Jimmy Carter and Ronald Reagan.To contact the author of this story: Cass R. Sunstein at csunstein1@bloomberg.netTo contact the editor responsible for this story: Katy Roberts at kroberts29@bloomberg.netThis column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.Cass R. Sunstein is a Bloomberg Opinion columnist. He is the author of “The Cost-Benefit Revolution” and a co-author of “Nudge: Improving Decisions About Health, Wealth and Happiness.”For more articles like this, please visit us at bloomberg.com/opinion©2019 Bloomberg L.P.