Im confused! Does the phone work with your o2 Sim? If so why do you need to unlock it?
You can usually guess if the phone was branded and by who by looking at the installed bloatware.
Yes, some later stock films have this option removed so that you can't unlock it. You will probably have to go to the original provider of the phone. In France unlocking is free (by law) if the phone is over 3 months old.
Roger