Apologies. Had a busy day yesterday and am currently back at work. When I get home later this evening I'll take a look at my install and try to help you out. I recall that I had to modify the Steam launch parameters and add the @AH64 (or whatever folder you have the AH-64 mod in) to the command line.
When I get home I'll get a copy of my Steam command line and maybe that will help...
Startup ParametersEDIT - One other thing I had to do (I think) was to copy an entire folder from ARMA2 to ARMA2 OA or vice versa..I can't remember which, but that solved the problem similar to what you had I think. Again, once I get home I think I'll have a better idea of what I had to do to get it working..
BeachAV8R