Antes de probar a revivirlo con el msmdownloadtool que es un poco complicado, puedes intentarlo con la rom de mauronofrio por fastboot, te dejo link:
https://drive.google.com/open?id=1QH...uXp1qf7zIeYN3t
HOW TO FLASH
Download the zip;
Unpack the zip in a folder;
Reboot the OnePlus 6 in fastboot mode (Power and volume-);
Connect the OnePlus 6 to PC;
Run flash-all.bat flasher you need;
Wait until the process end;
The phone will automatically reboot.