Can I buy an iphone 6 without contract and use my old sim?

So I currently own an iphone 5 with a contract from O2. IIm long over due for an upgrade but can't get one since my phone contract was made under my guardian s name, whom I no longer speak to(though this is another problem).
Anyway, IIm currently in Korea and my phone has practically given up after it's long service: data full no matter how much things I delete, screen freeze and crash, and sometimes turns itself off etc.
If I buy an iphone 6 here and inset my old sim from my current iphone 5, will it work fine and will O2 be ok with this?

Yes, the iphone5 and the iphone6 both take nano sim cards, the nano sim card from the iphone5 will fit into the iphone5. The services the nano sim card provides will be the same for the iphone6 as for the i phone5.