[OnePlus 6T  Questions & Answers] NoBodyUKnow:
It did clear some things up, thank you. However, I still end up at the Qualcomm CrashDump Screen of Death. this happens at the very end of the process, after the second load of LOS 17 and TWRP Installer, while rebooting to recovery from the TWRP .img. I'm going to list every step I take and all of the files I use. There are a couple of places where I'm not sure I'm doing the right thing, and/or the outcome of that step is not as expected.

1. Download all of files:

TWRP Q .img

TWRP Installer

OP OS 10.3.1 Fastboot ROM

lineage-17.1-20200223_133843-UNOFFICIAL-fajita.zip

First step is to flash Oxygen 10 and TWRP Recovery to both slots. I unpack the OOS Fastboot ROM zip on my laptop, boot to my 6t's bootloader and connect that to my laptop. After making sure ADB/Fastboot sees the device, I run "flash-all.bat" by clicking on it. This seems to load OOS just fine. The phone boots to OOS and I turn the phone off.

(I've done all of this several times and there are variants: I've tried becoming a developer and turning on the advanced boot options in OOS 10, but I don't think that matters, as long as I can get back to the bootloader or recovery. Let me know if that is incorrect).

At this point, your instructions say

"reboot to recovery, flash the OTA zip again."

I've tried moving the OOS fastboot zip file to my phone and flashing from TWRP, but that doesn't work. I get an error complaining of wrong format. It seems to me, a Fastboot ROM can only be flashed from adb/flashboot. Is that correct?

Therefore, I have been going back to the bootloader and re-flashing OOS 10 from adb fastboot again. After this step, contrary to your guidance above, the phone does indeed boot directly to OOS.... A loooong, what seems to be initial boot. I then turn off the phone.

I reboot to recovery again - and it is now the stock recovery - so I follow your guidance to factory reset from there. It does boot into OOS, and it is another looonnng boot. I then turn off the phone, return to the bootloader, flash the TWRP .img, install the TWRP installer and move on to the LOS 17 process linked to above.

Though I'm not sure it's necessary, I follow these steps from the beginning of he process:

Install 10.3.0 fastboot ROM again to start over, boot it once. Just to make sure everything is good.

- Reboot to fastboot.

- Boot the Q image of TWRP.

- Run the TWRP installer, reboot to recovery

Everything seems to work well, so I follow these steps, all from TWRP:

- Factory reset (This is the slider at the bottom in the wipe menu).

- Format data. (This is the button that says Format data in the wipe menu)

- Do NOT wipe cache, do not Wipe system, do NOT Wipe Dalvik / ART, do not add anything else, Just Factory Reset and Format Data. After doing those two Reboot to Recovery one more time. You must do this after formatting data.

I do all of the steps below, with no problems:

- Now transfer Lineage and TWRP to your phone, Windows / Linux should see TWRP fine as a media device when plugged in.

- Install Lineage

Just a note at this point, to highlight that this is done from TWRP, as opposed to using adb fastboot as with OOS10. This is what I've done with ROM's in the past; the fastboot ROM thing is new to me.

Next, I

- Run the TWRP Installer.

- Reboot to Recovery.

- Install Lineage (You need Lineage on both slots, so yes run it again)

- Run the TWRP Installer (yes again).

- Reboot to Recovery

This is where things go bad. In this reboot to recovery, the TWRP splash screen sits there for a long time, and then the Qualcomm CrashDump Screen of Death (QCDSoD) appears. If I hold the power button down for long time, the QCDSoD disappears and the phone reboots and starts to load LOS. Then, it switches to TWRP splash screen, and as before, end with the QCCDSoD.

I've been through this entire process maybe three times. Always the same outcome (you know what they say, about doing the same things and expecting different results...)

I did note, that on the OS Fastboot ROM page, there was some discussion of "Branded" phones, specifically T-Mobile phones. When I first got this phone, after loading TWRP, I swapped active slots. On the new, active slot, I booted the phone, and it was to a T-Mobile ROM. Not sure if that plays any part in all of this.

Appreciate your hanging with me. I'm going to rest my fingers :eek: