I really wouldn't bother. I have an o2 online contract, 7 months into it and counting' come Xmas & I'm outta there! When you have a problem, not if, you are forced to call their premium help line, they charge you 50p per minute. You can't take your handset (I'm on my 5th) to an o2 shop, they wont touch an online purchase. Corresponding via email is a nightmare and takes ages. You send a message, wait a week, they reply, bog standard reply, you reply wait another week and so it goes on! There website is atrocious Checking bills online is a minefield.
There are plenty of other deals out there, with other providers. I'm putting this down to a year's experience never to be repeated.