Fastboot Mode (also known as bootloader mode) is a diagnostic and flashing tool used primarily for advanced system modifications, such as unlocking the bootloader or installing custom recoveries. On the OPPO F11, accessing Fastboot is a bit different than on many other Android devices, because OPPO restricts direct Fastboot access on ColorOS.
Here’s a clear, step-by-step text on how to open Fastboot Mode on the OPPO F11. How to Open Fastboot Mode on OPPO F11