OnePlus
Add a Birthday to a Contact auf OnePlus
OnePlus Contacts stores a birthday as a Date field, not as a separate birthday field. If that field carries the wrong label, the date saves but Calendar and the dialer never recognize it as a birthday.
This mismatch happens often when a contact was imported from a SIM card, a vCard, or synced from an account that does not map its date fields the same way OxygenOS does. The date sits in the contact, but no reminder appears and no birthday cake icon shows in the contact list.
Add or fix the birthday field
Open the Contacts app ➔ Choose a contact ➔ Tap on Edit ➔ Tap on More fields ➔ Tap on Date (make sure Birthday is set as label) ➔ Enter the date of birth and tap Set ➔ Tap on Save.
The key step is the label check on the Date field. If it reads Anniversary, Other, or a custom label instead of Birthday, the reminder logic ignores it regardless of the date entered.
If the contact is synced from a Google account and birthdays still do not appear after this fix, the issue usually sits in the Google Calendar sync settings for that account rather than in the contact itself. Check the calendar sync options before repeating this process on every contact.