I get the same error, validating the files on steam does not appear to work. I've also got Arma 2, Arma 2: OA and all three DLCs.
I have tried validating files several times without it being fixed at all. I guess I'll try reinstalling it.
Edit: Forgot to mention that none of the buildings on Bystrica appear either. I've not yet tested the other two maps, but I assume they may have the same problems.
Edit 2: Reinstalling did nothing, I've also found a few other bugs too. No buildings in any of the 3 maps appear. Shooting at the new soldiers is sometimes weirdly delayed, making them seem unkillable, but they randomly fall over after a short while. Like I unloaded a full clip into one guy's head point-blank and he stood there for ages before dying.