Sig Medium has always been my favorite for our models, whether it be over solid wood or open bays. As far as the grades go I've always used either Sig or the K&S brand as my reference. I don't know what OOO would equate to but my understanding is that K&S "OO" is their lightest stuff, equal to Sig "Light", and primarily intended for Free Flight models and the like. From there, the K&S "GM" equals Sig "Medium" and their "SGM" equals Sig "Heavy".
As a side note, I'd stay away from Brodak's so-called silkspan. I tried a half dozen different packages of it a few years ago and the material was closer to a brown paper bag than silkspan, very heavy, difficult to work with, and it didn't take dope worth a damn. It didn't even work well over solid wood, after fighting it for a while I ended up just throwing it all away.