Vitamin C is sensitive to oxygen and the concentration of vitamin C in orange juice will thus decrease during exposure to oxygen. This process starts immediately after making orange juice.
The reaction also depends on temperature and acidity. In acid juices, such as orange juice, and at low temperatures the reaction will be relatively slow.
It is therefore recommended to drink orange juice within a few hours and keep it refrigerated as much as possible. On the other hand, the reaction is rather slow, which means that even after one day there is still some vitamin C present in the juice.
Commercial orange juice is packed in oxygen-tight packages and as long as the package remains unopened the concentration of vitamin C will be constant. After opening the situation is identical as described above for freshly made juice.