We know that when one buys an iPhone generally it is locked by a specific mobile network which consequently limits the use of the mobile device by allowing it to work only with the network's SIM card. So your iPhone will not recognize the SIM cards from other networks.
iPhone unlock varies depending on what network you are on : O2, Orange, Vodafone , AT&T, Rogers, etc. The unlock becomes a severe necessity when you want to enlarge the capacity of your iDevice, or if you are for instance a person who often has to be abroad, and sometimes have to be in countries where your SIM card becomes invalid as the network on your iPhone is not available in the country you are. But before we start, I want to give an answer to a question, whether it is legal to unlock an iPhone, as there are many iPhone users whom bothers that question. I must delight you as unlocking iPhone is legal, and you needn't refer to doubtful ways of unlocking. There are networks that will unlock your device and this service will cost you $20 utmost.
Here we are going to discuss some ways of unlocking O2 iPhone whether you are still in contract with the network or not. If you are under contract, I think it would not be prudent to wait for the contract to come to its end as 18 or 24 months is quite a long time to wait. So let's discover way outs that will free your device from restraints and will allow you to use the device on any network. You can refer to various iPhone unlock softwares to unlock your device and get rid of O2. There is no lack of softwares on the web.
First it seems it is good that you have a great variety of choice among softwares and if one doesn't work you can simply try another one but you had better keep in your mind that a fake software, containing viruses may cause great damages to your device. Before you activate a software learn whether you are well informed about the software. Learn what the software offers; get satisfying answers to your questions. Try to find people who have already used the software as it would be safer and more trustworthy to try an already used one. Anyway, O2 iPhone unlock is the most simple among those other networks mentioned above. And you can unlock your device hence with the help of O2.
If you do not want the services of O2 mobile network and you are still in contract there is an online O2 form you must visit and fill in. The process of unlocking will take two weeks the most. If you are a Pay Monthly customer you can unlock your device free of charge whenever you want though you are not allowed to cancel the contract. If you are a PAYG (Pay As You Go) customer then the unlocking costs £15 and if you decide to unlock your PAYG O2 iPhone during the first 12 months after signing the contract you will have to lose the free Web and WiFi.