Once you have A2, A2 OA installed, you'll want to run both so it can do it's initial setup. If you have the DLC, all of it should load besides the Czech Republic since you'll get an option to install that during your A2 OA install.
Next you'll need to get DayZ and DayZ Epoch through DayZ Launcher, which is practically the same as A3 Launcher. I'm not sure if the DayZ Mod on Steam will work, but you can try it out. But it might be better to download the mods through DayZ Launcher.
And that is pretty much it.