Have(Had?) the same problem. Bought CO pack, installed first game, second game - got an error like TS's one. Seems like the addon files are totally messed up, the mod requires stuff from Arma2 while it's incomplatible(not completeley) with Arma2:OA. I copied all the crap from \Steam\SteamApps\common\arma 2\AddOns into \Steam\SteamApps\common\arma 2 operation arrowhead\Expansion\Addons Seems fine now, at last.