OnePlus
Allow App Installs from Unknown Sources auf OnePlus
Android blocks installations from any source other than the Google Play Store by default. This protects you from apps that were not reviewed by Google, but it also stops legitimate APK files you download yourself, for example from a manufacturer website or a trusted developer.
On OnePlus phones running OxygenOS, this permission is granted per app rather than as one global switch. Whichever app you use to open the APK file, usually your browser or a file manager, needs its own approval before it can trigger an install.
Where to enable it
Open Settings ➔ Apps & notifications ➔ Special app access ➔ Install unknown apps. You will see a list of every app capable of opening installer files.
Tap the app you plan to use, for example your browser, then enable Allow from this source. If the toggle is missing or grayed out, the app itself may not support installer access, and you should pick a different app such as your file manager instead.
Repeat the process for every app you use to open APK files. Permission granted to one app does not carry over to another.
Once the source is trusted, downloading and opening the same APK file will trigger the normal install prompt instead of a warning.
Turn the setting back off afterward if you only needed it once. Leaving multiple sources permanently enabled increases the risk of installing unreviewed or harmful apps later.