The default calendar displayed on the OnePlus smartphone can be expanded by selecting an alternative calendar, for which we have the following options that can be activated with a tap of a finger:

Android 10 | OxygenOS 10.0
Step 1:  Open the  Calendar app
Step 2:  Open the  menu
Step 3:  Open the  Settings
Step 4:  Tap on  General
Step 5:  Tap on  Alternate calendar
Step 6:  Choose an  alternate calendar
  1. Open the Calendar app
  2. Open the menu
  3. Open the Settings
  4. Tap on General
  5. Tap on Alternate calendar
  6. Choose an alternate calendar

OnePlus Instructions