@Anonymous I don't believe there is a simple way. I'm not 100% certain about this, but I think you would either have to send it back to the seller to get them to unlock it with their O2 sim, or you would have to get an O2 Pay & Go sim, top it up with at least £20 and register it on the network, then apply for it to be unlocked. Either way, whatever bargain you thought you were getting by buying a locked phone isn't going to be as much of a bargain any longer.
Forum help thread here http://community.o2.co.uk/t5/Other-Products-Services/Unlocking-an-O2-phone-to-use-a-different-SIM-ca...
Seems we've missed this guide in the Knowledge Base. I'd really like to add it if you feel it's still very useful?
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