it's not clear to me if rendering an independent set of data on top of the unchanged OSM data layer (i.e. effectively render two maps, on with some transparent sections) counts as building on top of the OSM database
If you render OSM data with a unique map style, and produce an image, then you have made a "Produced Work" and you only have to attribute OSM, and nothing else. You can make it so people can't copy it if you'd like.
If you make a new database by combing OSM with something else, that new DB is ODbL.
Just showing something on top of OSM doesn't mean it's derived from OSM.
Re:Mapping the World
by Doctor_Fegg on Thursday January 08, 2015 @04:34AM
(#8855647)
It doesn't.
> You are free to copy, distribute, transmit and adapt our data, as long as you credit OpenStreetMap and its contributors. If you alter or build upon our data, you may distribute the result only under the same licence. The full legal code explains your rights and responsibilities.
http://www.openstreetmap.org/copyright
Although IANAL, and it's not clear to me if rendering an independent set of data on top of the unchanged OSM data layer (i.e. effectively render two maps, on with some transparent sections) counts as building on top of the OSM database. So maybe they wouldn't even need to use the ODbL for their own data.