Having a cup of OJ is not the same as having a serving of fruit. A whole orange contains many nutrients that aren't even found in a freshly squeezed cup of OJ. For example, the fiber in the whole fruit will do a better job of filling you up, and actually blocks some sugar from being absorbed. Juice (especially store-bought) contains a lot of sugar and lacks the antioxidants found in the pith of the orange (the white stringy parts).