If subtitle tracks are present in a video, they are not displayed on Oppo smartphone by default, but if the subtitles are desired, they can be enabled and adjusted in the accessibility settings.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Additional settings
Step 3:  Tap on  Accessibility
Step 4:  Tap on  Caption preferences
Step 5:  Enable or Disable  Captions
  1. Open the Settings
  2. Tap on Additional settings
  3. Tap on Accessibility
  4. Tap on Caption preferences
  5. Enable or Disable Captions

Oppo Instructions