The thing is, you have to install the OA into the original Arma2 directory in order to access the OA content and vice versa. BUT, once you do that, anything you build will require both games. SO, if you have buddies that only play with OA, they'll have to reinstall OA into the Arma2 directory in order to play your content. You will also have to relocate your DLC's to your Arma2 directory as well. (I believe) you can simply copy and paste their folders..(not certain though).