What finally worked for me on a Tmobile converted to International OP6T:
Download OP6T(OOS) latest firmware version full zip,
https://www.oneplus.com/support/soft...M1574156215016
Download magisk latest version.
Download latest stable PA Quartz ROM(links are down for now...)
Then assuming you already have TWRP Recovery installed, reboot to twrp and follow this quick guide...
The key here is to DO NOT reboot to system ever during this flashing process
Reboot to recovery "twrp"
Wipe>Swipe to factory reset>Format Data>YES
Reboot once again to twrp recovery, you should be on slot A
Plug the OP6t to your computer, transfer ALL three files to your device internal storage; OOS-AP ROM-MAGISK(if you used it)
*Get ready to flash away... follow this order to the letter...
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First objective to get latest OOS on both slots A/B:
Flash stock OOS(full twrp flashable zip)
Flash twrp installer
Reboot to twrp
Flash stock OOS
Flash twrp installer
Reboot to twrp
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Second objective to get LOS ROM on both slots A/B
Flash LOS
Flash twrp installer
Reboot to twrp
Flash LOS
Flash twrp installer
Reboot to twrp
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Flash Gapps
Flash magisk(Optional if you need it)
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*REBOOT to System.
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For Updating to newest, latest and greatest build...
Reboot to twrp recovery
Flash latest build
Reboot to recovery again
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flash Gapps
Magisk (optional if you use it.)
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*Reboot to system