Staff were completely unenthusiastic and clueless, ended walking out with an expensive phone that I returned 5 minutes later (I walked out, came to my senses outside the door, walked back in). The assistant who helped me didn't know anything she was talking about, I had to show her information off the O2 website. On the second time I went in (to return my phone) I spoke to the manager to cancel the new contract and he was being incredibly rude, in the end he got it sorted but I've now had to contact O2 assistance twice because he messed it up.
Ended up going to the store in my uni town and all the staff there are proactive, driven and genuinely kind. Got the new phone there in the end at discount which was "impossible" at the Oxford store, when I told him about the previous experience he listed a bunch of incentives they should have told me.
In short. don't go to this store even if you have no problems with O2, lazy staff and will rip you off