"About" is much more commonly used. I believe that "over" is sometimes used when discussing the opinions of several people about the same subject. For example, "There was much disagreement in their opinions over the sports competition." "Over" works better than "about" because their disagreements were not about the competition itself. Rather, their disagreements were related to the competition; for example, they preferred different athletes. However, I believe that even in this case, most people would say "about". If, however, their disagreement was about the fairness of the competition itself then you would have to say "about".