OTC reading glasses are the same prescription and they won't treat other eye problems, like astigmatism. So if you find difficult focusing on near objects, you'd better go to your doctor to have an eye test to see whether you have other eye problems. The doctor will prescrible a pair of reading glasses which suit you fine. Otherwise OTC reading glasses will do harm to your eyes if they are not prescribled for you.
Non prescription reading glasses are made with the same power in both lenses. They are suitable for people whose eyes have the same power in both eyes. However, most people have different unique prescription. Some people will even have sore eyes and dizziness when they wear non prescription reading glasses for a long time. Although they are much cheaper and can be purchased easily, it is better and more safe to have your eyes examined and wear prescription one.