The clock on the Always on Display can be changed on the Oppo smartphone from version ColorOS 7 (Android 10) onwards. In addition to digital clocks, the settings also offer analogue clocks in various styles and colours.

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:  Tap on  Clock style
Step 5:  Choose desired  Clock style
Step 6:  Tap on  Apply
  1. Open the Settings
  2. Tap on Display & brightness
  3. Tap on Always-On Display
  4. Tap on Clock style
  5. Choose desired Clock style
  6. Tap on Apply

Oppo Instructions