Use my UK iphone 6 in USA

Hello. I am moving from the UK to the USA in a couple of months. Can I use my factory unlocked UK iPhone 6 (Model A1586) there on local networks?

Is it a simple case of just changing nano sim card to local one?

At the moment I am on GiffGaff (which uses the O2 network) as I said, my phone is factory unlocked, no carrier apps etc pre installed.