By default, the volume keys are used to change the media volume, but alternatively, the Oppo smartphone keys can also be used to change the ringer volume.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Sound & vibration
Step 3:  Tap on  Volume button function
Step 4:  Choose between  Media volume  and  Ringtone volume
  1. Open the Settings
  2. Tap on Sound & vibration
  3. Tap on Volume button function
  4. Choose between Media volume and Ringtone volume

Oppo Instructions