The time or hour format is automatically recognized and set by the OnePlus smartphone based on the selected region.

To keep the clock displayed in the usual style, the hourly format can also be selected manually by disablingthe 24-hour format function, giving you the choice between A.M. or P.M.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  System
Step 3:  Tap on  Date & time
Step 4:  Enable or disable  Use 24-hour-format
  1. Open the Settings
  2. Tap on System
  3. Tap on Date & time
  4. Enable or disable Use 24-hour-format

OnePlus Instructions