Device: OnePlus Nord (avicii)
Recommended Recovery: https://pitchblackrecovery.com/avicii

Clean Flash:
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1. Boot to recovery

2. Flash the ROM.zip file from internal storage or USB OTG or via ADB sideload

3. Format data

4. Reboot


Dirty Flash:
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1. Reboot to recovery

2. Flash the ROM.zip file from internal storage or USB OTG or via ADB sideload

3. Reboot


OTA:
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1. Simply upgrade via ota updater (either if pushed or via local update import)