If we have added a birthday to our saved contacts, we can view the data in the Calendar app on the OnePlus smartphone so we don't forget an appointment.

Depending on whether multiple Google Accounts are linked to the OnePlus device, we can turn on/off the display of birthdays for each account individually.

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  Birthdays
Step 5:  Enable or disable  Contacts
  1. Open the Calendar app
  2. Open the menu
  3. Open the Settings
  4. Tap on Birthdays
  5. Enable or disable Contacts

OnePlus Instructions