Will my iPhone work in Germany?

Hey,

I'm currently living in the USA because I study here. I will return to Germany next year.

I was thinking about buying an iPhone 6 here because they are cheaper.

So my question is does somebody know if the phone will work in Germany when I unlock it here in the US?

I will have a sim card here in the US and I have one in Germany its from O2 but I don't know which one I will have here, yet.