Oppo
Add a Birthday to a Contact auf Oppo
ColorOS treats a birthday as an extra contact field, not as a calendar event. That is why the option is hidden under additional details instead of appearing on the main contact screen.
To add it, open Tap on Contacts ➔ Choose a Contact ➔ Tap on Edit contact ➔ Tap on More fields ➔ Tap on Significant date (Birthday). This last step reveals a date field that is otherwise not visible on the standard contact form.
Enter the date and confirm with Enter the Corresponding data and tap Set, then finish with Tap on Save to write the change back to the contact.
Once saved, the birthday only shows up when you open that specific contact entry. ColorOS does not push a notification, add the date to your calendar, or highlight upcoming birthdays anywhere else in the interface.
If you rely on reminders, this field alone will not help. In that case, add the birthday manually to your calendar app as a recurring event, since that is the only way to get an actual alert on this system.