Coll@teral

Hi there, I don’t own ARMA II. I want to start playing on a local server. I want to buy the right version of ARMA II. Now I’m confused as to which version to buy… do I buy 1) Arma II OA Standalone 2) Or, Arma II CO? I guess what i'm wanting to know is, if I buy OA Standalone, is there any content available in Arma II standard that wont be available in Arma II OA, that would prevent me playing with people just running Arma II (Standard)? Much thanks, i've searched the forums, but i haven't find an answer just yet.