Get into Fastboot or Recovery

adb reboot bootloader

adb reboot recovery

This applies to all OnePlus devices including OnePlus 6T:

• Power off the device.
• Press and hold Volume Up + Power button.
• Release the Power button when you see the OnePlus logo and keep holding the Volume Up button until you see the fastboot mode splash screen.

• Reboot to fastboot mode as shown above.
• In the fastboot mode, use the Volume keys to navigate to Recovery Mode and use the Power button to select it.
• Alternatively, with the device powered off, press and hold Volume Down + Power button until you see the OnePlus logo.