I had ARMA II already, so I just bought operation arrowhead (non-steam version)
I had to copy + paste the data files from
...\ARMA II - PC\AddOns
...\ARMA II Operation Arrowhead - PC\Expansion\Addons
to get the game working. I still get a single error message but the game seems to play fine. If you don't have ARMA 2 to begin with, just get combined ops.