Most Oppo smartphones come with Google's Chrome browser pre-installed, but if you want to use a different browser, you will need to set it as your default browser after installation.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Settings
Step 2:  Tap on  App management
Step 3:  Tap on  Default app
Step 4:  Tap on  Browser app
Step 5:  Choose desired  App
  1. Open the Settings
  2. Tap on App management
  3. Tap on Default app
  4. Tap on Browser app
  5. Choose desired App

Oppo Instructions