Unlock

How to unlock bootloader for OnePlus smart Phone - OnePlus Support - OnePlus (United States)

Fastboot

Get the Google USB Driver  |  Android Developers
Download and Install Android USB Drivers for popular OEMs (xda-developers.com)

MSMTool

[OP8][OOS TMO 55CB] Unbrick tool to restore your device to OxygenOS | XDA Forums (xda-developers.com)
UPDATE: OnePlus 8 IN2017 T-Mobile to International (with Bootloader Re-LOCK) | XDA Forums (xda-developers.com)
instantnoodlet_15_O.06_200411.zip | by Some_Random_Username for OnePlus 8 (androidfilehost.com)
https://onepluscommunityserver.com/list/Unbrick_Tools/Qualcomm%20drivers.7z