[OnePlus 7T Guides, News, & Discussion] hypercoyote:
nolimit78 pieced together a lot of good info and I just wanted to post a revision on that with some clarification for those who may not be too familiar with this process. The link to this is at the bottom.

For the record, I performed this on a OnePlus 7T (Model HD1907), Build 10.0.12.HD63CB, Android Version 10

1. Update System Software

Ensure the phone is on the latest system software (which should be at least 10.0.6) to make the conversion easier. This will keep you from having to step through an intermediate firmware version:

In the Settings, go to System > System Update and click 'Check for update' or resume/install an ongoing update

2. SIM Unlock (Carrier Unlock)

This requires putting in a request with T-Mobile and took about 3 days for me.

Request SIM unlock from T-Mobile on Facebook (message them from here: https://www.facebook.com/TMobile/)

They will ask for your IMEI and phone number, then verify that you are not on a contract.

I've heard results may vary with this, but I didn't have any issue and I did two phones

Once they confirm it is unlocked, performed the following to unlock on the phone:

In Settings, go to Wi-Fi & internet > SIM & network > Network Unlock.

Choose unlock type: Permanent Unlock

3. Boot Unlock (Bootloader Unlock)

Enable Developer Settings:

Go to Settings > About Phone

Tap on "Build Number" 7 times and you will see a message at the bottom of the screen the developer options are enabled

Use Developer Settings to enable OEM Unlock:

Go to Settings > System > Developer options

Enable "OEM unlocking" (shown disabled below)

Get IMEI by dialing *#06# or in Settings > About Phone > Status

Enter fastboot mode to get the unlock code:

If you haven't already, install the OnePlus 7T USB Drivers by connecting it to your PC while the phone is powered on normally. It will mount a drive for "OnePlus Drivers" from which you can run the driver installation. Note: For me, the installer got stuck at the 'executing commands' step and I had to restart the PC and reinstall, then it installed quickly.

Download “fastboot_windows.zip” and unzip it on your computer.

Power off the OnePlus 7T.

Press and hold the “Volume +” and “Volume -” and then "Power" keys until the “fastboot mode” menu appears

With this menu open on the phone, connect the phone to your computer via USB

Open a command prompt (not Powershell) on Windows, switch to the fastboot directory and run it using the following commands:

Note: If it says 'waiting for device', then you need to install the OnePlus USB drivers or determine why the driver isn't working

cd <fastboot directory>
fastboot.exe oem get_unlock_code

Submit the the information found above at this link and wait 7 days for them to send you the unlock file: https://www.oneplus.com/unlock_token

THESE NEXT STEPS WILL WIPE YOUR USER DATA! BE SURE YOU HAVE BACKED UP YOUR DATA BEFORE DOING THIS!

After receiving the unlock_code.bin file from OnePlus, put them in the fastboot directory and run the following commands:

fastboot flash cust-unlock unlock_code.bin
fastboot oem unlock

A warning will appear on the screen after the last command explaining that once the boot loader is unlocked and a custom OS is installed, they cannot verify the integrity of the system. Use the “Volume +” and “Volume -” buttons to highlight the unlock the bootloader option, then press "Power" button to select it. The phone will wipe your user data and then reboot. It may take a while to reboot while it displays the "T-Mobile" loading screen. Let it sit at least 10 minutes before trying to reboot.

Note: Your device will display a warning at boot until you re-lock the bootloader. Many have recommended not re-locking the bootloader due to various issues with it.

4. Installing Global Firmware

For this step, you *might* need adb tools installed. I think that everything you need is contained in the ROM archive from step 1 below, but if you get an error, download SDK Platform Tools for Windows from this link and install it.

Download the newest *GLOBAL-OnePlus7TOxygen*.zip from this link: https://sourceforge.net/projects/fastbootroms/files/OnePlus 7T/

Extract the contents of the zip file for use later.

Return to the fastboot menu on the phone:

Power off the OnePlus 7T.

Press and hold the “Volume +” and “Volume -” and then "Power" keys until the “fastboot mode” menu appears

Connect your Phone to the PC via USB.

Open a command prompt and change into the directory for the *GLOBAL-OnePlus7TOxygen*.zip

From the command prompt, run the flash-all.bat file

Note: At one point, the phone will reboot and mine showed a menu with a language selection screen instead of the normal Fastboot menu. DON'T TOUCH THE PHONE, BE SURE TO STAY AT THE LANGUAGE SELECTION SCREEN. I clicked through it and went back to the fastboot menu and that made everything else fail.

FYI, I received the following errors, but it seemed to work:

# Error 1D:\Downloads\OnePlus 7T Unlock\Global Fastboot ROM (Latest)\10.0.13-GLOBAL-OnePlus7TOxygen_14.O.19_OTA_019_all_2009281709_1026-FASTBOOT>fastboot flash reserve reserve.img
Sending 'reserve' (250164 KB) OKAY [ 5.863s]
Writing 'reserve' FAILED (remote: '(reserve_b) No such partition')
fastboot: error: Command failed

# Error 2
D:\Downloads\OnePlus 7T Unlock\Global Fastboot ROM (Latest)\10.0.13-GLOBAL-OnePlus7TOxygen_14.O.19_OTA_019_all_2009281709_1026-FASTBOOT>fastboot flash opproduct opproduct.img
Invalid sparse file format at header magic
Sending sparse 'opproduct_b' 1/1 (391796 KB) OKAY [ 9.447s]
Writing 'opproduct_b' OKAY [ 3.180s]
Finished. Total time: 14.376s

# Error 3
D:\Downloads\OnePlus 7T Unlock\Global Fastboot ROM (Latest)\10.0.13-GLOBAL-OnePlus7TOxygen_14.O.19_OTA_019_all_2009281709_1026-FASTBOOT>fastboot flash system system.img
Invalid sparse file format at header magic
Resizing 'system_b' OKAY [ 0.005s]
Sending sparse 'system_b' 1/5 (523136 KB) OKAY [ 12.658s]
Writing 'system_b' OKAY [ 3.076s]
Sending sparse 'system_b' 2/5 (523148 KB) OKAY [ 12.544s]
Writing 'system_b' OKAY [ 3.071s]
Sending sparse 'system_b' 3/5 (523640 KB) OKAY [ 12.597s]
Writing 'system_b' OKAY [ 3.074s]
Sending sparse 'system_b' 4/5 (524256 KB) OKAY [ 12.526s]
Writing 'system_b' OKAY [ 3.083s]
Sending sparse 'system_b' 5/5 (197932 KB) OKAY [ 4.786s]
Writing 'system_b' OKAY [ 1.364s]
Finished. Total time: 77.293s

# Error 4
D:\Downloads\OnePlus 7T Unlock\Global Fastboot ROM (Latest)\10.0.13-GLOBAL-OnePlus7TOxygen_14.O.19_OTA_019_all_2009281709_1026-FASTBOOT>fastboot flash vendor vendor.img
Invalid sparse file format at header magic
Resizing 'vendor_b' OKAY [ 0.005s]
Sending sparse 'vendor_b' 1/2 (523681 KB) OKAY [ 12.737s]
Writing 'vendor_b' OKAY [ 3.065s]
Sending sparse 'vendor_b' 2/2 (427876 KB) OKAY [ 10.438s]
Writing 'vendor_b' OKAY [ 2.626s]
Finished. Total time: 32.395s

Once the phone reboots, you should see the OnePlus boot-up screen instead of the T-Mobile one and all should be working.

References

Retrieve unlock token: https://www.oneplus.com/unlock_token

Unlocking bootloader: https://support.oneplus.com/app/ans...-to-unlock-bootloader-for-oneplus-smart-phone

Install process: https://xdaforums.com/t/guide-wip-t-mobile-brand-conversion-to-rooted-global-firmware.4019495/