Just having a look around modding websites for arma I found a World War 2 full convert mod. After installing it very easily playing the mod its very very good plenty of new weapons, animations and tanks ;). You can play this online but I think it would require a lot of work but its awesome as a standalone mod.
http://forums.bistudio.com/showthread.php?t=117585
For steam users they will have to install via common folder just change the default root.
Looks like it needs Combined Ops, probably for the European maps, so probably not usable by those with only OA.
I had a go with this mod a while back, and it was barely working.
Possibly worth a second look now it's had some more development time thrown at it though.
It seems to be working OK with me firing range works all the weapons work well the only problem is that the campaign's say cannot load world or there is missing items but other than that its fine. I dont have combined ops just OP and BAF it has its own maps like omaha
After sometime finally getting stuff to install all you need is ARMA2 and ARMA2OA to get this to run perfectly and I will tell you that its so much fun! The sounds are amazing once a huge battle starts plus with just iron sights getting the chance to move in is vital seriously I would of paid for this mod!