Hi! I had the same problem a few hours ago. If you bought Arma II OA by Steam, just right click on Arma II OA in Steam, properties, Local Files, and check cache Files integrity (sorry; in french it's "Vérifier l'intégrité du cache", i don't know how to translate correctly), the thirth option. In my case, Steam redownloaded 4Gb of Arma II OA, and the error "No entry..." disappaired. Hope it'll help :)