On the Oppo smartphone, you can change the ringtone in general, but also for individual contacts and contact groups, and you can use the default tones or your own ringtone for the group.

Android 10 | ColorOS 7.0
Step 1:  Open the  Contacts -App
Step 2:  Open the  Menu
Step 3:  Tap on  My Groups
Step 4:  Tap on  Group  you want
Step 5:  Open the  Menu
Step 6:  Tap on  Ringtone
Step 7:  Choose desired  Ringtone
  1. Open the Contacts-App
  2. Open the Menu
  3. Tap on My Groups
  4. Tap on Group you want
  5. Open the Menu
  6. Tap on Ringtone
  7. Choose desired Ringtone

Oppo Instructions