To start with, OCI and PIO are two kinds of cards that are obtainable by an Indian citizen. You must have the India passport or have parents or grandparents that are related to India. If you came from a territory that is a part of India, you could also possess the OCI or PIO, provided you are not a citizen of some certain places such as Pakistan, Bangladesh, and Sri Lanka. The PIO actually covers like four generations back, which makes it of a broader aspect than OCI, which has some limitations.
You are basically entitled to a PIO card if you are an Indian citizen, that is, if you have your parent, grandparent, or great grandparent residing permanently in India. You are also eligible for your PIO if you are an Indian citizen, but you are away from the country with your spouse, but you are not eligible for the OCI card. The OCI card does not extend beyond your grandparent. In addition, the PIO lasts for only 15 years, while the OCI lasts for a lifetime.