Arma 2 Operation Arrowhead doesnt recognise Arma 2 orignal

[BadWolf33 1]

but ive been playing Opertation arrowhead for a year, and I just bought arma 2 the orginal version, but OA doesnt have arma 2 in its expansion pack addons in the menu, or editor, as if OA doesnt see the orginal Arma 2, how can i fix this?

[Nicholas 5]

I'm pretty sure you need to install ArmA II first, then install Operation Arrowhead to combine them.