I disagree, in my opinion, all C's in prereqs for pharmacy school should be retaken, if you think you can get an A. If you can't get an A, don't try. You should take another class like Biochem, in addition to retaking the Ochem over, not as a replacement.
I would say that replacements are only valid, if you take a class that is next in the series. I remember going to an pharm school open house and the admission person said to take the next class in the series. So if you got a C in Ochem, take the next class which is Advance Organic Chemistry, or if you got a C in Intro to Physiology, take the next physiology class at your school. Also realize though that this is very risky because if you couldn't get an A in the intro Ochem or physio class, you might get screwed in the more advanced class.