Do I really need Arma II: CO instead of just Arma II?

[chirf 0]

On this website it says I need Arma II: CO

When I go on steam, the only thing that looks like Arma II: CO is the combined operations.

Must I really spend $30 on that instead of $20 on the original Arma II?

Is there a reason I'd need Arma II: CO over the other?

[landstriker 9]

You need both, yes. "Operation Arrowhead" is a standalone game, but you still need to have both Arma 2 and OA installed in order to run the mod.

[sorophx 25]

you need ArmA CO. BUT that actually means that you need Operation Arrowhead, which is only 20$ in Sprocket.

ArmA2 original doesn't have a lot of things that are required to play the mod (Venoms, some weapons, backpacks etc.), on the other hand OA doesn't have Chernarus. that's why you absolutely need CO to play DayZ, but, like I said, you can get it by combining OA and ArmA Free