Do I need to download both Arma 2 and Arma 2 OA having run arma 2 once before on another PC?
You do need the games to run atleast once, not sure in what machine matters or not but it shouldn't.
Arma 2 be on another partition and still give me access to the correct things in OA
It should be no problem at all aslong as you link them properly with SteamMover.