Can I then play Day Z? Or do I have to actually buy Combined Operations.

Yeah, you can just buy Operation Arrowhead if you already have ArmA II. I'm not sure if the version of the games you have matters (as in steam or non-steam versions.)

You could splice the free Arma 2 version with a paid copy of Arrowhead on steam. Then play Dayz. So if that works, then your Paid arma 2 and paid Arrowhead will combine fine be they both steam, neither steam or a mix.

Those two are all you need. Just how you will need to look up. Had a friend splice free arma with paid steam arrowhead. And plays Dayz with us.

Ya I have the paid version of ARMA 2 from steam. Thanks fuys.