[OnePlus 6 Questions & Answers] dolorem:
Hello,

I am gathering information here for people having problems with flashing Android 9 ROMs to their Oneplus 6 phones. I am among them. I've browsed through a big bunch of threads and still can't get to a working custom ROM OS.

I've noticed that there are very differing instructions on this forum on how to do the flashing properly. Here are some:

https://xdaforums.com/showpost.php?p=77987633&postcount=2370

https://xdaforums.com/showpost.php?p=77114327&postcount=127

https://xdaforums.com/showpost.php?p=78021561&postcount=2404

Part of the problems seem to be because in older threads, guides and videos people flash older ROM versions, older Magisk etc. that might contribute to different results.

To give a context, I'll start with my own story.

Status before my attempt

Phone model ONEPLUS A6003 = the EU/US model (not A6000 if that makes any difference)

Phone unlocked already (fastboot oem unlock reports: "Device already : unlocked!" whenever I check)

The OOS I am talking about == OnePlus6Oxygen_22_OTA_014_all_1810301355_5a3cd838ad89482d.zip

The TWRP I am talking about = twrp-3.2.3-x_blu_spark_v9.86_op6.zip

The Magisk I am talking about == Magisk 17.1.

My attempt

fastboot booted to blu_spark TWRP img (TWRP SHOWS CURRENT SLOT = A)

Wiped System, Data and Dalvik / ART Cache

Flashed OOS (from Oneplus downloads) and TWRP (OOS FLASH 1/2 DONE)

Booted to OOS up to the pin code screen

Booted back to TWRP (TWRP SHOWS CURRENT SLOT = B)

Wiped System, Data and Dalvik / ART Cache

Flashed OOS (from Oneplus downloads) and (OOS FLASH 2/2 DONE)

Booted to OOS up to the pin code screen

Booted back to TWRP (TWRP SHOWS CURRENT SLOT = A)

I am pretty certain that up to this point everything is done properly: we have a booting OOS on both A and B system partitions and also the A/B boot partitions contain TWRP.

Where it all goes wrong

This is where the instructions diverge. Some say that:

I should now wipe System, Data and Dalvik / ART Cache

I should just do a factory reset wipe without wiping System

After doing either of these steps all the guides agree on me having to flash the ROM and TWRP to the current slot.

So, after this step, if we forget about Gapps, Magisk etc. We should have a working ROM in the current slot. But when I boot to system, I am always thrown back to TWRP and can never boot any custom ROM.

Questions

I have a couple of questions I'd love to get answers to clarify and build a definitive guide for flashing:

Since the wipes (wiping System or doing just factory reset) in TWRP do not touch the Vendor partition, am I correct in assuming that the reason why OOS flashing to both slots is required because the OOS rom provides content for the Vendor partition and possibly updates radio firmware or something else?

If my previous assumption is correct, can't we assume that once we have flashed OOS in slots A/B their contents are permanently and we do not need to start EACH custom ROM flash by flashing OOS twice?

I would really appreciate clear answers to these two questions and especially hints on where my procedure goes wrong (why am I left with a phone that will always and only boot back to TWRP).