Believe it or not Michael Oher has really stepped up in pass pro over the last couple of years, the problem with him is his run blocking which is really surprising to me because that's what his main strength was. He had some ATROCIOUS games last year as a run blocker, and the trend continued into the play offs, he has fixed some of his mental lapses though at least.
Also KO struggled on the perimeter because he was a rookie, he did a fine job in a lot of games. Against the Giants he pretty much stonewalled JPP, and had some other good games against other elite pass rushers. In fact, he seemed to raise his game against the better pass rushers, then had some bad games against the lesser quality of players like Antwan Barnes. So maybe that's more of a mental issue rather than a physical issue. Don't forget that in college he came up against Aldon Smith a bunch of times and always had good games against him.
So I think giving KO a shot at LT should be considered, and if Castillo thinks that he can be a good LT, then he must have something. I think what you see is what you're pretty much going to get out of Oher, a good RT, would probably be a pro bowl guard, and a serviceable LT.
If Harbaugh really doesn't want McKinnie back, I'd bring Max Starks over for a 1/2 year deal, and have him compete with KO at LT, and see how it goes, if KO blows it up there, you can play him there and have a pretty good swing tackle, and if KO really struggles in pre season etc, you can move him back to guard, and re evaluate who you want at LT long term while starting Starks.