Are you fully patched?
Using any custom mods/addons?
I am fully patched as I am aware. I am using a few modifications, I'll list them below.
ACR Troops. Woodland ACR Camo.
ACE 2:RU. Advanced Combat Enviroment 2. Russian Faction.
ACE 2:USNavy. Advanced Combat Enviroment 2. US Navy with Marine Corps.
M2 Bradley Reskin.
SAF - Singapore Leopard2A4.
British FV432 "Bulldog" APC.
British Ridgback PPV.
Landrover 101 FC.
Patch Version: 1.52
That is all the mods I have at the moment.
Considering your system spec -which is very good- any falloff of FPS should be happening in ArmA II, and not in :OA.
How many monitors are you feeding?
Pull up both your ArmA2.cfg & ArmA2OA.cfg and check for anything obvious between the settings.
No doubt you've already gone over this but I'll ask anyway...tried updating (or rolling back) your Catalyst drivers?
No, I never mess with my drivers, I haven't ever touched a driver in my life, lol. I checked the ArmA2.cfg and ArmA2OA.cfg and there is nothing apart.