Original ArmA 2 OA missions are in missions_e.pbo file. You would need you extract this file, find mission you wand and open that mission in editor and add some playable roles. Then save as multiplayer mission. I have once remade some mission from OFP campaign to be playable for two players.
If you want to read about gaming tacticks, try Dslyecxi's ArmA2 Tactics, Techniques, & Procedures Guide:
http://ttp2.dslyecxi.com/