This sometimes happens after a failed reset. Force shut down the phone (Power + Volume Up) and repeat Method 2. Ensure you select "Wipe data" and not "Wipe cache" by mistake.
The phone will restart and take 2–5 minutes to complete the reset. How to Hard Reset OPPO F3
A simple restart (soft reset) is not a factory reset. If someone tells you to "hard reset your OPPO F3" due to a password issue, they mean Method 2 above. This sometimes happens after a failed reset
Ensure your battery is at least 40% charged to prevent the device from shutting down during the process. How to Hard Reset OPPO F3