GuideForOplusUdon.MD

Those guides are applicable to every 11R/Ace 2 model:

Navigation

ALL DATA WILL BE DELETED AFTER UNLOCKING THE BOOTLOADER

Enable OEM unlocking "Advanced settings" -> "For developers"
Download SDK Platform Tools and extract it to the root of drive C.
Download ADB setup 1.4.3
Extract adb setup and open file.
Click Y on All
Connect the phone to the computer and enable file transfers
A USB debugging window from this computer will appear on the screen. Click OK
Open a command prompt in this folder
adb reboot bootloader - phone rebooted in bootloader
fastboot flashing unlock - bootloader unlock (if u see waiting fo any device use Google for installing fastboot driver)
Select Unlock the bootloader by volume buttons and press power to confirm.

• Prerequisites

Make sure your device is unlocked (fastboot flashing unlock and fastboot flashing unlock_critical).
Backup your important data before proceeding.

• Clean flash

reboot to bootloader
fastboot flash boot boot.img
fastboot flash vendor_boot vendor_boot.img
fastboot flash recovery recovery.img
fastboot reboot recovery
Select Factory Reset
Apply update -> adb sideload
adb sideload rom.zip
After 47% recovery ask you to reboot recovery. Ignore and click NO.
Go back to the main menu.
Select Reboot
If u need Magisk,Gapps(for Vanilla builds) and etc. reboot to recovery and adb sideload filename.zip

• Dirty flash (Update)

Go to Recovery
Apply update -> adb sideload
adb sideload rom.zip
After 47% recovery ask you to reboot recovery. Ignore and click NO.
Go back to the main menu.
Select Reboot
If u need Magisk,Gapps(for Vanilla builds) and etc. reboot to recovery and adb sideload filename.zip

Fixing error 7 while flashing a custom firmware

Make sure fastboot flashing unlock_critical before flashing stock
Download files OOS COS
Extract the zip file in any folder
Connect the device & make sure you have all drivers install in your system
Run the .bat file
The window will close automatically after installation and the phone will reboot into the system
After booting into system flash rollback OOS & COS via localupdate (find in group #local_install_oos))
Install latest update (Full package)