06-11-2019 10:38 - edited 06-11-2019 10:41
Do you mean unlock? If so, follow the guide here Guide: Unlocking an O2 phone to use a different SIM card
Most O2 phones are sold unlocked now, so try another network sim first.
on 06-11-2019 10:39
There is a guide for this:
on 06-11-2019 11:14
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)