The lad in my local Tesco store's mobile phone shop told me today that it is not possible to put an O2 sim card into a Tesco Mobile phone. (A shame as I wanted to buy a Tesco mobile PAYG phone and ditch the SIM card, and put my wife's O2 sim card into it.)
But is that advice right?

Yea, although it might depend on the handset make and model

Yes, he's right and in my experience it's on all Tesco Mobile phones. They need to be unlocked to take an O2 sim. However, a Tesco Mobile sim will work in an O2 phone with no probs, apparently.

Ok - Thanks guys. A pity.

Depending on the phone an extra tenner to get it unlocked may be worthwhile.....I bought a phone from the local Tesco shop and then got it unlocked 10 minutes later at a small store in the Tesco building......