For those who own (and run) Combined Ops (ArmA2 & Op. Arrowhead) on Steam, something seems to have been changed by BIS/Valve with the way that CO is run. Essentially, the option for starting CO has been deprecated and is no longer required. Starting Op. Arrowhead should now automagically start CO. Unfortunately, there seem to be a small number of issues (that have had some of us scratching our heads for a night or so).
Firstly, unless it has been done already, Steam seems to have changed ArmA2 and OA over to their new format (along with a lot of other games), so you'll need to try and run both first, to get the stuff converted... should take about 5 minutes, maybe a little longer.
Once that's done, you'll need to do the first time set up stuff again, like you did when you first installed and ran CO... Run ArmA2 at least once to... do whatever it is that that did. Then, run OA and it should run CO. Check the expansions menu, or the list ojn the main menu for the ArmA2 entry, if it's there you should be golden.
If not, and if only OA runs and there's no A2 content, then we have a problem. Luckily, it should be easy to solve, and is probably due to mods in your modline (Launch Options). Delete the -mod= and every @mod name... load NO MODS through that option. Instead, the Expansions menu on the Main menu should now work... Go into it, and enable the mods you desire, disable the mods you don't want, restart the game (or it should offer to restart itself).
Removing ALL mods from the mod line is a bit of overkill, but it's the easiest way to ensure you can get CO running, without 1/2 an hour of testing or so. The problem seems to be if there are @folders in the A2 directory that have the same name as @folders in the OA directory, and the -mod= line is in launch options (even if those duplicated @folders are NOT being loaded), something stumbles when the game tries to get the list of A2 mods and the list of OA mods (to be added to the expansions menu) and finds duplicate entries. When there are no mods loaded through that way (-mod=) there doesn't seem to be a problem (for myself and the others I've spoken to, at least).
If you own and run CO through Steam, you'd be well advised to check you can get it running no problem prior to the FOLK session today, or TT on Tuesday. If you own CO through Steam, but don't run it through Steam (use a launcher etc.), then I don't think there's any issue.
1. Convert A2 and OA on Steam, by trying to run them individually.
2. Run ArmA2 once.
3. Clear the -mod= line from Launch Options if it's there.
4. Run OA. CO should be run.
5. Organise/Enable/Disable mods through the Expansions menu.
6. Die face down in the mud in order to stroke the egos our incompetent, vainglorious, inept, callous leadership cadre, and keep afloat their delusions of tactical genius FOR GLORY!!
This isn't from exhaustive experimentation or study or anything like that, just from how a couple of us on the RPS Steam chat room have been able to get it to run. If you've anything to add... add it below. If I've missed anything, let me know.