By default, the Oppo Smartphone's alarm clock rings for 5 minutes until it turns off automatically, and can be adjusted to ring at the following times

Android 10 | ColorOS 7.0
Step 1:  Open the  Clock -App
Step 2:  Open the  Menu
Step 3:  Tap on  Settings
Step 4:  Tap on  Ring duration
Step 5:  Choose desired  Duration
  1. Open the Clock-App
  2. Open the Menu
  3. Tap on Settings
  4. Tap on Ring duration
  5. Choose desired Duration

Oppo Instructions