[OnePlus 6T  Guides, News, & Discussion] Extreme_Ninja2099:
This is a bit late but why not. THIS IS A COMPILATION OF ALL THE OS METHODS YOU NEED FOR THE OP6/6T.

ENJOY my guide :)

DISCLAIMER:

*I am not responsible for bricked devices, damage to your device*

Sorry for a long list of methods, some work for others and some don't. But I recommend to pick a method and stick to it. Maybe read the part at the top as some have notes about mine or someone else's experience/recommendation. Don't skip steps and follow each step.

If stuck, just ask and I or anyone else will try and help 🤗

I am here to post methods for anyone who is interested/need help. I have tested these myself and helped others who may be struggling :)

Download

The guide has loads of files/ zips/ tools to use/flash/install. But no worries as everything can be downloaded and there are links AT THE BOTTOM OF THIS THREAD (NOT COMMENTS) to where to download:

SDK Platform Tools (Fastboot tools) from Developer Android

TWRP from mauronofrio

Magisk from topjohnwu

Gapps (Google Apps) from NikGapps. I recommend omni for ROMs that don't include Gapps.

Fastboot ROM for OP6 + OP6T from mauronofrio

MSMDownload tools for OP6 + OP6T from Some_Random_Username

IMPORTANT NOTES!

YOU REQUIRE Platform Tools (Fastboot Tools) IN ORDER TO USE THE METHODS IN THIS GUIDE AS TWRP NEEDS TO BE INSTALLED!

YOU NEED TO HAVE A UNLOCKED BOOTLOADER AS WELL!

For GAPPS, u can use the same or a updated version of the package. But has to be the same type u used to install it like (stock, full, Omni, basic, etc). ONLY FLASH/INSTALL IF the RESTORE SCRIPT DIDN'T SHOW UP during a DIRTY FLASH/UPDATE AND ONLY FLASH AFTER REBOOTING TO RECOVERY AFTER FLASHING/INSTALLING A OS/BUILD ZIP FILE!

How to get Platform Tools

These tools are used to install TWRP, to unlock the bootloader of your device and is needed to use the METHODS in this guide.

Download the platform tools.zip of your software platform (Mac, Windows, Linux)

Extract the platform-tools.zip

To use it go into your platform-tools folder

Press shift + right-click and choose command prompt or PowerShell

Connect your device when it's in fastboot mode and done

How to install TWRP

Sadly this Guide is for Windows, but other platforms like Mac or Linux can be found online.

YOU CAN USE THIS METHOD TO INSTALL A NEWER VERSION OF TWRP TOO

You need to have downloaded Platform Tools which the link is attached below.

Transfer the TWRP.img file into the platform-tools folder

Now transfer TWRP.zip into your device's internal storage. (connect -> allow file transfer -> drag and drop files)

If you want root download and transfer magisk.apk and the link is supplied below. Also, RENAME from .apk to .zip

Now we must boot into fastboot mode. (Either do it with the advanced reboot in developer options or power off device -> boot recovery Vol - + power -> boot to bootloader or power off device -> press Vol Up + Vol down + power)

Now connect your device to a Windows device

Press shift + right-click. Choose command prompt or PowerShell

Enter "fastboot devices" and your device should be displayed if it's connected

Type "fastboot boot twrp" file name (do twrp + press tab) and press enter

Then you should be booted into TWRP

Enter the password and swipe to allow modifications

Press install and find your TWRP.zip file

Swipe to confirm Flash

If you want root do this

Press the back arrow and find your magisk.zip

Swipe to confirm Flash

Reboot to the system and done

Otherwise, just reboot to system. Up to u

Installing a Custom ROM

Install Custom ROM on OP6T from OOS

Make sure u have BACKED UP your device and have the latest OOS installed in both slots. If u do, go to Method 3.

If u have finalize.zip, go to Method 2.

Method 1

If u don't have OOS installed in both slots or finalize.zip, use this method.

Transfer OOS.zip file, TWRP.zip, Build.zip (Custom ROM zip file) into internal storage.

Boot to Bootloader

Boot TWRP

Flash OOS + TWRP

reboot recovery

Flash OOS + TWRP

Reboot recovery

Flash Build.zip (Custom ROM zip file)

Flash TWRP

If the ROM doesn't have GApps included do this

Reboot to recovery

Flash Gapps if not included

Format data

Reboot to system

Otherwise, just Format data -> reboot to system. Up to u

If you want root do this

Reboot to recovery

Flash Magisk

Reboot to system and done

Otherwise, just reboot to system. Up to u

Method 2

If u have finalize.zip, do this method. Using finalize is way quicker as there's less flashing and rebooting.

Transfer OOS.zip file, finalize.zip, TWRP.zip, Build.zip (Custom ROM zip file) into internal storage.

Boot to Bootloader

Boot to TWRP

Flash OOS + finalize

Boot to Bootloader

Boot to TWRP

Flash Build.zip (Custom ROM zip file)

Flash TWRP

If the ROM doesn't have GApps included do this

Reboot to recovery

Flash Gapps if not included

Format data

Reboot to system

Otherwise, just Format data -> reboot to system. Up to u

If you want root do this

Reboot to recovery

Flash Magisk

Reboot to system and done

Otherwise, just reboot to system. Up to u

Method 3

Once you have installed OOS in both slots do this part.

Transfer TWRP.zip, Build.zip (Custom ROM zip file) into internal storage.

Boot to Bootloader

Boot to TWRP

Flash Build.zip (Custom ROM zip file)

Flash TWRP

If the ROM doesn't have GApps included do this

Reboot to recovery

Flash Gapps if not included

Format data

Reboot to system

Otherwise, just Format data -> reboot to system. Up to u

If you want root do this

Reboot to recovery

Flash Magisk

Reboot to system and done

Otherwise, just reboot to system. Up to u

Installing Custom Rom not from OOS

You can use finalize method or just flash OOS + TWRP way. The finalize way is there for some who like less flashing and rebooting.

Using the methods in

Install Custom ROM on OP6T from OOS

Using the old way, use Method 1

Using finalize.zip, use Method 2

Remember to factory reset (wipe, but no need to wipe internal storage)

Backing Up OOS and Custom ROM

Make sure u have installed the latest TWRP version from mauronofrio

You should use TitaniumBackup or Migrate to back up your apps and contacts.

Also, have a copy of your Internal Storage folders if u have important files.

TWRP Backup backs up your OS like Apps, settings and set up, but not internal storage.

NEVER BACKUP SYSTEM AND VENDOR! ONLY SYSTEM.IMG or VENDOR.IMG IF YOU HAVE TO!

Restoring it makes going back way easier like you have never left :)

Or you can go manually and try and restore everything, but why do that when you got TWRP?

I do both ways of backing up

Backing up OOS

Boot TWRP recovery

Choose Backup

Choose boot, data. YOU DO NOT NEED TO BACKUP System.img and Vendor.img!

Swipe to backup

Once done, either boot to system or mount

Transfer the whole TWRP folder onto a storage device

Done

Backing up a Custom ROM

Boot TWRP recovery

Choose Backup

Choose boot, data, system.img, vendor.img

Swipe to backup

Once done, either boot to system or mount

Transfer the whole TWRP folder onto a storage device

Done

Restoring OOS and Custom ROM

Restoring OOS TWRP Backup

To restore OOS use either fastboot ROM or MSM. Fastboot is the fastest and easiest way if u have a TWRP backup which there's a method to how to make one in this guide. BUT if you are restoring to go back to stock OOS then use MSM

I use Fastboot ROM restore for that too as we are restoring our data anyways and also for switching/installing a custom ROM. Much easier in my experience.

DOWNLOAD FULL ZIP OF OOS version of your TWRP backup otherwise you could risk doing the whole restore method again OR setup from SCRATCH! IF you aren't restoring a TWRP backup, I recommend either TRANSFER or DOWNLOAD via WiFi or oxygen updater, a UPDATED version of OOS AFTER doing either fastboot ROM or MSM restore methods. As they restore a old version of OOS!

Restoring using Fastboot ROM

Download fastboot ROM of your choice using the link in the Download Link section.

Extract/ un-zip

Boot your device into fastboot mode (Either do it with the advanced reboot in developer options or power off device -> boot recovery Vol - + power -> boot to bootloader or power off device -> press Vol Up + Vol down + power)

Click flash-all.bat

Restoring using MSM Download Tool

Download latest MSM version of your choice using the link in the Download Link section.

Extract/ un-zip

Click on MSM tool .exe file

Turn off device

HOLD BOTH volume buttons

Connect your phone

Wait for your device to be connected and it should show as green

Click verify to check and press start

Wait till phone reboots

The setup device and u can carrying on restoring your device if u don't want a unlock bootloader or return to what your device was like before.

Go into developer options

Allow OEM unlocking

Reboot to bootloader

Enter "fastboot devices" to check your device is connected

Enter "fastboot oem unlock"

Swipe for yes

Then it will reboot again

continue to the next method

For both fastboot and MSM

Once booted into system, set it up and update to OOS version of your TWRP backup.

Download the full zip using the link in the Download Link section.

Transfer the OOS .zip to internal storage.

Go into settings -> system update -> Update via local upgrade -> find zip file and press install

Do this twice to make sure you have installed that version of OOS in both slots

Boot to bootloader

Boot TWRP

Install TWRP

Reboot recovery

Transfer whole TWRP Folder into internal storage

Restore only boot and data

Swipe to restore

If you have backed up your internal storage folders do this

Reboot to recovery

Transfer all your internal storage folders

Reboot to system and done

Otherwise, just reboot to system. U can update OOS as well if it's outdated. The method is below to update OOS with ROOT. Up to u

RESTORING TWRP BACKUP HAS ISSUES ON THESE ROMs

CrDroid (Method 1 + 2 don't work). Method 3 works (install Gapps if the ROM doesn't have it!)

Restoring a Custom ROM TWRP backup 1

I tried using this method, but it didn't work for me.

You can use this method to switch ROMs too

Boot TWRP Recovery

Flash OOS + Finalize

Boot to bootloader, boot TWRP

Flash TWRP

Reboot Recovery

Transfer twrp backup

Restore System.img, Vendor.img and boot

Boot to system and setup system

Reboot to recovery

Restore data

Reboot system. Done

I had an issue with this method where rebooting system it was from fresh and my data didn't restore. If this happened to you, then do method 2 (

Restoring a Custom ROM TWRP Backup 2)

Restoring a Custom ROM TWRP Backup 2

The first method didn't work for me. But this method did

Restore fresh like you're going back to OOS.

Choose one method. Fastboot or MSM method.

I prefer the fastboot method.

Download fastboot ROM using the link in the Download Link section.

Extract/ un-zip

Boot your device into fastboot mode (Either do it with the advanced reboot in developer options or power off device -> boot recovery Vol - + power -> boot to bootloader or power off device -> press Vol Up + Vol down + power)

Click flash-all.bat

Once booted into system, set it up briefly and DO NOT RESTORE ANYTHING.

Boot to bootloader

Boot TWRP

Install TWRP

Reboot recovery

Transfer whole TWRP Folder into internal storage

Restore everything (System.img, vendor.img, boot.img and data)

Swipe to restore. If data couldn't restore then boot system, setup system, boot TWRP and restore data.

Reboot system. The first reboot might go dodgy and it did for me.

Reboot system and transfer all your internal storage stuff

done

If this did not work still. Then do the 3rd method...

Restoring a Custom ROM TWRP backup 3

If the previous 2 methods has failed for you. Then try this.

Restore fresh like you're going back to OOS.

Choose one method. Fastboot or MSM method.

I prefer the fastboot method.

Download fastboot ROM using the link in the Download Link section.

Extract/ un-zip

Boot your device into fastboot mode (Either do it with the advanced reboot in developer options or power off device -> boot recovery Vol - + power -> boot to bootloader or power off device -> press Vol Up + Vol down + power)

Click flash-all.bat

Once booted into system, set it up and update to OOS version of your TWRP backup.

Download the full zip using the link in the Download Link section.

Transfer the OOS .zip to internal storage.

Go into settings -> system update -> Update via local upgrade -> find zip file and press install

Do this twice to make sure you have installed that version of OOS in both slots

Transfer TWRP.zip, Build.zip (Custom ROM zip file) into internal storage.

Boot to bootloader

Boot TWRP

Install TWRP

Reboot to Recovery

Flash Build.zip (Custom ROM zip file)

Flash TWRP

If the ROM doesn't have GApps included do this

Reboot to recovery

Flash Gapps if not included

Format data

Otherwise, just Format data -> reboot to recovery. Up to u

Reboot Recovery

Transfer twrp backup

Restore data

Reboot system. Done

If it still fails, repeat steps 1 to 15. But restore everything and you should be back on your custom ROM like you never left.

Updating OOS and Custom ROM

Update OOS with Root (Magisk)

Always do a TWRP backup before updating your OS in case

Download the full zip file of the latest OOS, transfer the zip file to internal storage NOT IN ANY FOLDER

Download and transfer twrp.zip file to internal storage

Go into settings, system update, update via local upgrade and choose the file

Wait till the installation has finished. If failed TRY AGAIN!

DO NOT REBOOT!

Go into Magisk Manager, modules

Find the twrp zip file and install TWRP

DO NOT REBOOT!

Go into Magisk Home

Press install

Press install via inactive slot

Reboot system and done

You will reboot into the next slot

You don’t have to repeat the installation to install the latest OOS in the other slot. But you should and I use this method many times and has never let me down.

Updating OOS without root

Go to System Update -> find the update install and reboot.

Or download the zip -> transfer to internal storage -> update via local upgrade -> reboot and done

Update a Custom ROM

Method 1

Always do a TWRP backup before updating your OS in case

Use this method just to update the ROM.

Boot recovery

Flash new version + twrp

Reboot recovery

Flash new version + twrp

If the ROM doesn't have GApps included,

The restore script should show up during the installation of the new ROM.zip

BUT if the restore script DIDN'T pop up do this

Reboot to recovery

Flash Gapps if not included

Reboot to system

Otherwise, reboot to system. Up to u

If you want root do this

Reboot to recovery

Flash Magisk

Reboot to system and done

Otherwise, just reboot to system. Up to u

Method 2

This method uses finalize.zip

Boot recovery

Flash new version + finalize + twrp

If the ROM doesn't have GApps included,

The restore script should show up during the installation of the new ROM.zip

BUT if the restore script DIDN'T pop up do this

Reboot to recovery

Flash Gapps if not included

Reboot to system

Otherwise, reboot to system. Up to u

If you want root do this

Reboot to recovery

Flash Magisk

Reboot to system and done

Otherwise, just reboot to system. Up to u

Updating vendor

Use Method 1 and 2 for ROMs with Gapps. Use Method 3 for ROMs with no Gapps included.

Method 1

If u haven’t updated your Custom ROM and your Vendor… You can use this method to update both or just the vendor.

Boot recovery

Flash OOS + twrp

Reboot recovery

Flash OOS + twrp

Reboot recovery

Flash Custom ROM + twrp

Reboot recovery

Flash Custom ROM + twrp

If you want root do this

Reboot to recovery

Flash Magisk

Reboot to system and done

Otherwise, just reboot to system

Method 2

This method uses finalize.zip

Boot recovery

Flash OOS + finalize + twrp

Reboot recovery

Flash Custom ROM + finalize + twrp

Otherwise, reboot to system. Up to u

If you want root do this

Reboot to recovery

Flash Magisk

Reboot to system and done

Otherwise, just reboot to system

Method 3

This method is for ROMs with no Gapps included

You can use this method as well for updating to a vendor that has a NEWER ANDROID version like from OOS 10 to OOS 11 vendor.

Boot recovery

Flash OOS + Custom ROM + twrp

Reboot recovery

Flash OOS + Custom ROM + twrp

If u flashed GApps, the restore script should show up during the installation of the ROM.zip

If the GApps script didn't show up do this

Otherwise, skip step 3

Flash Magisk if u want root, otherwise skip step 2

Reboot to recovery

Flash Magisk

Flash Gapps

Reboot to system and done

Download Links:

SDK Platform Tools (Fastboot Tools) Here

OOS versions for OP6 (under Signed Flashable zips) Here

OOS versions for OP6T (under Signed Flashable zips) Here

TWRP for OP6 Here

TWRP for OP6T Here

Magisk Here

NikGapps Here

Stock Fastboot ROM for OP6 Here

Stock Fastboot ROM for OP6T Here

MSMDownloadTool for OP6 Here

MSMDownloadTool for OP6T Here

TitanumBackup App Here

Migrate App Here

Credits:

@mauronofrio For his amazing work on the TWRP recovery and Stock Fastboot ROMs for OP6/6T

@topjohnwu For his work on Magisk

@Nikgapps For their work on creating Gapps

@Some_Random_Username For his Collection of MSMDownloadTool for OP6/6T

@OPTeam For the collection of OOS zips for OP6/6T

@dreamwhite For the method (Method 3) of updating the vendor which works wonders for ROMs with no GApps included