I am not into this Steam stuff but the commandline you are giving him starts only OA, not CO.
Isn't it so that with the ArmA II Game Launcher
you do not have to add A2 or OA to the command line at all. You just specify which mods you want to load, select to start CO (you can choose between A2 / OA / CO) and the launcher will take care of things for you?
But like I said, I am not into this Steam stuff (I hate Steam) so maybe the launcher I linked does not work that easily with the Steam versions of the game.
@[FR] Black Mamba
It does not matter where you place CBA in the commandline, it is made so it always loads last (and it should load last, not first!).