I've got the same problem, ArmA 2 running great, OA, while only margnially slower than ArmA 2, suffers from sudden drops in framerate resp. momentary freezes for half a second every, say 10 to 20 seconds.
Windows 7 64 bit here, too, AMD Athlon 64 X2 5600+, and ATI 4850 (no sli). Using Catalyst 10.6 (I'll try 10.3 again tonight, although that means I have to do something about that stupid flash player... meh). Also copied the whole ArmA 2 installation to a different disk, no change.