Once upon a time the OCI used to be a 'U' type visa in the form of a sticker in your passport which you had to carry with you even if the passport had expired. The current OCI card came about after the merger of the PIO and OCI programs and is the visa sticker in a handy booklet format that doesn't need the old passport (once the new passport is registered in the system). Since they switched to a central database (which is queried by the airlines at checkin) to record the passport number the empty pages to endorse/update things are no longer required/used.