We can always change the sound for notifications of events or appointments from the Calendar app on the OnePlus smartphone.

Here we can use the pre-installed notification or ringtones of the system, but it is also possible to set a custom ringtone such as mp3-file, for example.

Android 10 | OxygenOS 10.0
Step 1:  Open  Calendar
Step 2:  Open the  menu
Step 3:  Open the  Settings
Step 4:  Tap on  General
Step 5:  Tap on  Calendar notifications
Step 6:  Tap on  Advanced
Step 7:  Tap on  Sound
Step 8:  Choose a  ringtone
  1. Open Calendar
  2. Open the menu
  3. Open the Settings
  4. Tap on General
  5. Tap on Calendar notifications
  6. Tap on Advanced
  7. Tap on Sound
  8. Choose a ringtone

OnePlus Instructions