Things I have hit / ideas:
1) Try setting the minSDK as Android Marshmellow API 23
2) Check that you are using the Oculus(Android) package not the Oculus(Desktop) package.
3) After setting the platform to Android, sometimes this has undone setting the Player Settings->XR enabled, so double-check that this is still set correctly.
4) As mentioned, use OpenGL ES 3.0, not Vulcan.
Just had the same black screen problem 10 AUG 2020.
1. upgrade minimum Android level to API23
2. Oculus XR Plugin set it to 1.4 on the upper left (maybe you need to enable experimental assets)
Took like 2-3 hours.