I want to buy an iphone, but....

Hi people, im new on this forum and had previously had 2 ipnones but got rid of them. I am now looking at purchasing a new iphone 3gs off a friend of mine.. Problem is im on o2 and the phone is on vodafone.... now although the phone is still boxed and the plastic still on the boxing will this be ok to put my o2 sim straight into it or will it have to be unlocked? help please as i dont want to waste my money....

Thank you in advance.