Oppo smartphones with an OLED display have an Always On Display function.

The Always on Display permanently or scheduled shows the time, date, battery charge level and from ColorOS 7 (Android 10) also notifications.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Display & brightness
Step 3:  Tap on  Always-On Display
Step 4:  Enable or Disable  Always-On Display
  1. Open the Settings
  2. Tap on Display & brightness
  3. Tap on Always-On Display
  4. Enable or Disable Always-On Display

Oppo Instructions