When the SIM card is delivered, it is protected by the provider by a four-digit PIN number as standard to prevent unauthorized access.
Since the standard PIN consists of an arbitrary combination of numbers, it is usually rather difficult to remember and a change of PIN is desired.

The PIN change can be made directly on the OnePlus smartphone and is stored directly on the SIM card, providing cross-device protection of the data contained there.

Note: If a PIN is entered incorrectly three times in a row, it is first blocked.
It can then be unblocked by entering the eight-digit PUK number.

This can be found in the corresponding SIM card documents.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  Security & lock screen
Step 3:  Tap on  SIM card lock
Step 4:  Choose the  SIM card   (Dual SIM)
Step 5:  Tap on  Change SIM PIN
Step 6:  Enter the current/old  PIN  and tap  OK
Step 7:  Enter the new  PIN  and tap  OK
Step 8:  Enter the new  PIN  again and tap  OK
  1. Open the Settings
  2. Tap on Security & lock screen
  3. Tap on SIM card lock
  4. Choose the SIM card (Dual SIM)
  5. Tap on Change SIM PIN
  6. Enter the current/old PIN and tap OK
  7. Enter the new PIN and tap OK
  8. Enter the new PIN again and tap OK

OnePlus Instructions