OnePlus Nord N100 (oneplus-billie2)

Contributors

 * ichernev
 * BenTheTechGuy

Via hardware buttons
Hold, , and at boot.

Via recovery mode

 * If you're not already in recovery mode, run  or hold  and  at boot.
 * English -> Advanced -> Reboot to fastboot -> Reboot to fastboot (confirm)

Via "Advanced Reboot"

 * make sure Settings -> System -> Developer Tools -> Advanced Reboot is enabled
 * hold power and choose bootloader

Installation
This device is in pmaports, but the downstream kernel doesn't boot and now fails to compile. There is work to mainline the device, but it's still early days. The source and a blog with some info are available below.