If you are not receiving the OTA notification, you can download the verified ColorOS 3.1 (Nougat) firmware from the official Oppo Support website and install it manually.
Select "Install from Storage Device" (or "Install from SD Card"). Select the downloaded ROM zip file. Wait for the process to complete, and select . Troubleshooting: Oppo F3 Update Issues oppo f3 nougat 711 update download verified
: Get the System Upgrade Tool on your PC from the OPPO Global ROM Page. If you are not receiving the OTA notification,