You need an app to force the phone to install the older zip file. Download the OnePlus ROM Upgrade APK .
: Since newer OxygenOS versions (12+) often hide the "Local Upgrade" option, you may need to install the OnePlus Local Update Tool . Step 2: Prepare the Package Downgrade Oneplus 9 To Android 11
Downgrading will wipe all data on your phone. Please read the prerequisites carefully before proceeding. You need an app to force the phone
Ensure your phone has at least 50% charge to prevent it from shutting down mid-process . Downgrade Oneplus 9 To Android 11