[OnePlus 7 Pro Questions & Answers] OblixioN:
Device: OP7pro purchased from China with a default HydrogenOS installed (GM21 build).

Goal: to install TWRP

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Update 1:

Using 10.0.2GM21AA, I managed to convert the HydrogenOS to OxygenOS.

But, after "fastboot boot twrp.img", the phone still freezes at reboot with oneplus logo and "Fastboot Mode" on the screen.

I have tried:

update to 10.0.3, freeze

update to 10.3.0, freeze

rollback to OxygenOS 9.5.1GM21AA, "fastboot devices" no longer detect my cell phone.

update to 9.5.7GM21AA, "fastboot devices" detects no devices.

rollback to 9.5.1, "fastboot devices" detects no devices.

Regarding "fastboot devices" unable to detect devices:

The adb and fastboot tool has been updated to the latest version.

The phone can boot normally into OxygenOS, stock recovery, and bootloader.

adb devices would detect the phone within OxygenOS.

"fastboot devices" was able to detect my other oneplus phone in bootloader.

"sudo fastboot devices" produced no different outcome.

"dmseg" shows that my computer could recognize my other oneplus phone in bootloader as a usb devices, but does not recognize the newly connected OP7P in bootloader.

Now, I am suspecting that my bootloader was somehow broken.

Frustrating.

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Original Post:

Steps I have taken so far:

Phase 1. fastboot oem unlock (smooth and easy)

Phase 2. Install TWRP (and fail)

I have read several guides suggesting, within bootloader, "fastboot flash boot twrp.img" or "fastboot boot twrp.img" like a breeze.

I understand that OP7pro uses AB partition, and the purpose is to boot into a temporary twrp and then to install a permenant twrp.

But for me, "fastboot flash boot twrp.img" causes infinite bootloader loop; while "fastboot boot twrp.img" causes a freeze at reboot with oneplus logo and "Fastboot Mode" on the screen.

The latter can be remedied with a reboot, while the former mandates a factory reset using the default oneplus recovery.

Phase 3. Convert HydrogenOS to OxygenOS (and still fail)

After some further studies and several nights' attempts, I noticed that Hydrogen OS implements an old version of fastboot, so fastboot boot into a temporary twrp is not supported.

Instead, I should upgrade into OxygenOS first, but I could not.

The OxygenOS was downloaded from the official site

version 10.3.0.GM57AA released on Jan 6th 2020. md5 checked. this zip file is adb push'd to /sdcard/

Others seem to breeze through the conversion from HOS to OOS without the slightest challenge.

But as soon as I select "local upgrade" from HydrogenOS default update installer and the OxygenOS zip file, I would receive an error prompt "upgrade failed" with the progression bar stopping at 0%.

This doesn't seem to be caused by the version of my HOS, so far, I have tried:

3.1 Updating HOS to the lastest version 10.0.2.GM21 released on Dec 12th 2019, and installing OOS 10.3.0.GM57AA, "upgrade failed"

3.2 With version 10.0.2.GM21, perform a system and cache wipe using the default OP recovery, and installing OOS 10.3.0.GM57AA, "upgrade failed"

3.3 With version 10.0.2.GM21, perform a system and cache wipe as well as a factory reset using the default OP recovery, and installing OOS 10.3.0.GM57AA, "upgrade failed"

3.4 With version 10.0.2.GM21, perform a full downgrade using the default HOS update installer, and installing OOS 10.3.0.GM57AA, "upgrade failed".

The downgrade rom was the official downgrade tool downloaded from:

download.h2os.com/Rollback/fulldowngrade_wipe_18821_190416_2308.zip

I feel I have exhausted all my options.

Is my device malfunctioning or I did something foundamentally wrong in the process?

Would an official North America Oneplus repair service accept this device and help me converting it to OOS?