OSM data is licensed via the Open Database License (ODbL) - of particular interest to the folks in this consortium is the `share-alike` provision of that license, which posits that you can make derivative databases of the data (such as augmenting it with other data/metadata) as long as you assign the ODbL to the derivative data.
> "...if you improve our data and then distribute it, you need to share your improvements with the general public at no charge. A painless way to do that is to contribute your improvements directly back to OpenStreetMap."
This is a problem if you have proprietary data that you want to use to augment OSM. Some interpretation here: https://wiki.osmfoundation.org/wiki/Licence/Licence_and_Lega...?
tl;dr from that document:
> "...if you improve our data and then distribute it, you need to share your improvements with the general public at no charge. A painless way to do that is to contribute your improvements directly back to OpenStreetMap."