Can I buy a Pay-as-you-go mobile [from Tesco] and have it work on a O2 contract Sim?

[Math]

If not, what would I have to do to get it working?

 

[TaraJune]

you need to get your phone unlock by a small phone shops

 

[jocoolguy]

You need to Unlock the Mobile Phone from Tesco network United Kingdom(UK) then use the Mobile phone with any GSM Network Sim card like United Kingdom(UK) O2 .....You can get the unlock code for O2 Network locked Phones at here Wickedunlock.com/supported-phones/rs11wp7/ and unlock it then use with any GSM network Sim card.....If you buy a unlocked phone means you no need to unlock it again.