Jaws2Blood, I suspect that the answer to PVPscene's question is that you are trying to use a command-line launch as per vanilla. That will not work.
Use the shortcut called "Launch Arma2 OA Beta Patch" inside the main Arma directory ( or more conveniently, copy it to your desktop).
The betas work more like addons than patches. That's why if you look at the command line in the "Properties' window (right-click on the shortcut) you'll see something like
Steam probably different, but it doesn't matter. Just double-click the shortcut and the beta will launch.
"G:\ARMA II\Expansion\beta\arma2oa.exe" -beta=Expansion\beta;Expansion\beta\Expansion -nosplash
You can add any mods you want through the "Expansions" option on the main in-game menu.