3rd parties can sell O2 contracts - but sometimes can offer better deals on the handset cost, cashback deals or other initial discounts (free insurance) which O2 can't offer themselves. Your monthly bill and contract is with O2, but your phone is provided by whichever company you take the contract out through.
The buying power of the 3rd parties means they negotiate their own prices for the handsets from manufacturers, and some probably also get some form of reward from O2 for referring a customer and signing them up to a 12-24month contract.
