It depends on your tariff / volt benefits, as some include o2 Travel Inclusive, and some you need to pay £5 a day for usage.
Have a look at the roaming Megaguide here Guide: Roaming MegaGuide
It's actually £6 a day now if you use O2 Travel @Andy37
Also be aware that O2 throttles it's data speeds to 0.5Mbs in the US. If you're going to be there for any length of time it's worth considering getting a local Pay & Go sim from AT&T or T-Mobile if you need faster data speeds.