if i buy a phone from an o2 shop would i be able to put a giffgaff sim in?

[moolah72]

i enquired in an o2 shop about buying a galaxy ace phone,and about putting a giffgaff sim in,the shop assistant said it not possible to do that,i did mention giff gaff is part of o2,he replied that giffgaff only use their phone masts,dont know who to believe as other forums say you can put a giffgaff sim in a o2 purchased phone,if so any idea why shop assistant is adamant you cant put giffgaff sim in it,any help i would appreciate thank you