[rmc]

Due to ODbL licence they can't mix OSM data directly with other data.

The ODbL has no restrictions on mixing, unlike the creative commons “no derivies” clause. The ODbL just says that you need to share those changes, and that new database, with everyone else. If we give you OSM data, you should give us back the data you combine with it.

If the other dataset doesn't allow that, then the restriction is from that dataset's licence, not from OSM's ODbL.