Can the i phone 4s on 3 network be used on 02 net work !

2 x samsung galaxy s3 for sale £360 each.
2x Samsung galaxy S3 smartphones for sale! 360 each or 700 both. 1 x white, 1 x Pebble Items Brand new and still in box, sealed. Selling as i dont need them anymore. Brand New, Grab yourself a bargain. Prefer Pickup.