how can i unlock my Nokia N73 to all networks?

It is currently on Orange but I am wanting to unlock it onto O2 network.

Any mobile can be unlocked. People will tell you about codes and using your imei number to get an unbloking code. Don't listen.
You need to take your phone to a mobile repair show, should be one local to you and ask for them to "unlock" it, for the n73 we would normally charge £10. So expect to pay between 10 and 20, no more than 20.