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The volume for the alarm clock can only be changed generally on the Oppo-Smartphone under ColorOS, but not for each individual alarm clock set.

Android 11 | ColorOS 11.0
Step 1: Tap on  Settings
Step 2: Tap on  Sound & vibration
Step 3: Set the  Alarms volume
  1. Tap on Settings
  2. Tap on Sound & vibration
  3. Set the Alarms volume
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Oppo Instructions