O2 offers an excellent deal with their Simplicity tariff(less than 10 pounds for 150 minutes and 300 texts, and you can quit at any time). However, the reception in Oxford is completely ridiculous(in many pubs, there’s no reception at all, for instance). I’d recommend to stay away from O2(I’ve been with them for almost a year before I quit) and use a different provider, at least if you’re living in Oxford.