For h815 models only. Do not process for other models.
First copy the downloaded files to your phone.
To perform ROM installation on the phone by unlocking the Bootloader and root
Perform the twrp operations. (LG G4 bootloader unlock, root and TWRP install)
After that, switch the phone on in Twrp mode.
Select all the operations needed in the backup section of Twrp and scroll down to the bottom bar. At this stage, you will be able to back up the whole system of the phone and take precautions just in case.
After the backup phase is finished, follow the steps of Twrp's wipe > Advanced wipe. In this section, select cache, Dalvik cache, system and data options and clear them.
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After clearing, go back to the twrp main menu and enter install. Flash ROM and Gapps package files respectively.
Reboot the phone by entering the reboot section of Twrp and pressing system.
LG G4 Android 8.1 OREO ROM experience.
LG G4 Android 8.1 Oreo ROM Download
LG G4 Android 8.x Oreo GApss Package Download