In the process of installing LineageOS on my OnePlus 6 A6003, I accidentally pressed "reboot to system" when there was no OS installed. How can I get back to TWRP? I've tried holding Power+Vol Down but it just boots with the oneplus logo then unlocked bootloader warning then black screen with the status LED on.
You need to press vol-up + power and then briefly (1-2 secs) let go of the power button (don't let go of the vol-up button yet!) and hold it down again until you see the TWRP splash screen.
I solved my problem with this tool.
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.