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OLC does not provide "advice" to the President that he's free to accept or reject. Its legal opinions are binding on the President. If OLC says that "X is illegal," then -- absent some extraordinary actions (such as the President formally rejecting the OLC opinion, which almost never happens) -- then the formal position of the Executive Branch is that "X is illegal."
In that case, the OLC could conveivably issue an opinion stating that since the Bush OLC issued an opinion that torture (or illegal surveilance) is legal, Johnsen could issue an opinion that would bar the Obama administration from prosecuting those that obeyed Presidential orders which were made legal by that opinion.
Is that correct?
If so, that would give Obama an "out", if he wants it, and keep him from ordering the DoJ from investigating and prosecuting those persons who engaged in illegal activities, including illegal wiretaps and torture among others.