MMS messages are sent in a similar way to an SMS message: as soon as a multimedia file (photo, video, contact (vCard), etc.) is attached to the SMS message, it is automatically converted into an MMS on the Oppo smartphone.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Messages -App
Step 2:  Open  Conversation  you want
Step 3:  Tap on  Add
Step 4:  Choose desired  File type
Step 5:  Tap on  Photos
Step 6:  Select an  Album
Step 7:  Select  Photo  you want
Step 8:  Tap on  Send
  1. Open the Messages-App
  2. Open Conversation you want
  3. Tap on Add
  4. Choose desired File type
  5. Tap on Photos
  6. Select an Album
  7. Select Photo you want
  8. Tap on Send

Oppo Instructions