Basically Operation Arrowhead is ArmA2 with some features of the ACE2 mod included in a desert environment. It may appear this is a small achievment, but, trust me, it isn't.
The game is gorgeous! Even more realistic, more assimilating, more complex, demanding and, hell, more amusing than ever! And, of course, there are tons of addons coming out every day, making AmrA2 OA virtually infinite.
I strongly suggest to buy both games or some sort of collector's edition, since many features and units needs the original ArmA2.