I bought the Arma 2 OA but is very satisfied with this game... Firstly, it crashes the whole Freakin time and I doubt it's because I am running on a Win 7 - 64 Bit OS. I have a AMD Athlon 2 DC with 4gig ram and Radeon 512, this can also not be the problem because the specs says Minimum Req is aat 256 with 1gig....
Frequent crashes are usually related to bad drivers.
What is your graphic card? Is it better than the Ati X1800?
Did you installed the latest drivers for your graphic card?
Are your OS is up to date?
Did you installed all the Arma II patches by the order?