In the nvidia Control Pannel, under Manage 3D Settings and then Program Settings tab, is D:OS listed? Does it help if you click on Add and browse to the '..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe' program file?
I'm not sure if/where the equivalent would be in the ATI Catalyst Control Center.
I've added D:OS in ATI CCC, but it doesn't helped. Devs should look at this.
