[OnePlus 6T  Guides, News, & Discussion] Striatum_bdr:
UPDATING WITHOUT ROOT​

If your bootloader is locked (meaning no root no TWRP, factory state), the update process is trivial, OOS (or Oxygen Updater if you use it) will grab a patch file (around 100MB max usually) and will apply it and will ask you to reboot. Everybody should be able to do this as it’s the normal way every phone on earth updates itself… If major changes are present sometimes you’ll have the whole ROM file. It doesn’t matter anyway it leads to the same destination.

If your bootloader is unlocked but without root, the only difference is that OOS (or Oxygen Updater) will download the WHOLE ROM (around 1.8GB) and will apply it then ask for rebooting

UPDATING WITH ROOT (+/- TWRP)​

Whole ROM update is mandatory. You can wait for System Update to download it or grab manually the zip file or use Oxygen updater. You then go in Settings > System > System Update > clic on the upper right cogwheel then choose ‘Local Update’ and select the zip file (If you download the OTA .zip file manually you must place it in the root of user data, like /data/media/0/ i.e the user internal storage, or eventually in the .OTA folder if there's one. If you use Oxygen Updater or System automatic donwload it will be right placed).

The following update method is the preferred one (even if you have TWRP, recommended by some TWRP devs) due to frequent changes in partitions by OnePlus

apply the ROM update via Settings > System > System Update > Local Update

DON’T REBOOT

Open Magisk Manager

[If you have TWRP, go in Modules menu, clic on ‘+’ and choose latest TWRP installer zip file on your phone. Don’t use Retention A/B Script Magisk Module I had issues with it] [then stay in Magisk Manager and do the next step]

On main screen of the app clic on ‘install’ next to Magisk paragraph

On the pop up menu confirm ’Install’

On the next pop up menu clic on the last entry: install on inactive slot (after OTA update)

Then only reboot.

The order can be changed (the most important thing to preserve is Magisk, TWRP is easy to flash back):

Update without taking care of TWRP, just Magisk part ("install to other slot after OTA thing")

Once rebooted to updated system, flash TWRP as a module with Magisk Manager

Install Magisk again using Magisk Manager ("direct install" option)

There are infinite debates about uninstalling or not Magisk Modules before updating. Most of the times nothing to touch, sometimes there could be issues, instead of uninstalling perhaps just inactivate them by precaution before updating.

Note: If you are rooted and run an OOS version older than 10.3.1, the above method should work, but you can also update your ROM to latest stable without taking care of root/TWRP and then root again as described in post #2

If you want to try TWRP way:

Reboot to TWRP

Flash OOS update

Flash TWRP installer

Reboot to Recovery

Flash Magisk

Reboot System

IF YOU ARE INTO A SOFT BRICK​

Don't forget the Vol+/Vol-/Power_button combo that can save you from a non responding phone...

If you can boot in fastboot mode, flash the stock ROM with fastboot. A little trick is to edit the flash-all-partitions**.bat and remove ‘-w’ (-w means that the partition will be wiped before flashing) from the script, it will prevent personal data wipe. Or flash manually all partitions one by one, without ‘-w’ flag evidently. Some partitions can’t be flashed with fastboot, for that once booted back to OOS, download and install by ‘local update’ method the same ROM version you’ve just restored with fastboot (see the linked thread to know why, emojis issues for example sometimes).

If you can’t go in fastboot mode, use Oneplus MSM tool. See there and there.