Can you use an o2 sim in a 3 locked phone?

[K9KAT]

Hey guys im lookin to buy the k800i and ive found its much cheaper on 3. Someone told me that you can use o2 sims in a locked 3 phone without unlocking, is this true?

[mcdaid123]

No you will need to get the phone unlocked first b4 u try and use a o2 sim or any other networks for that matter