Locus Tweak allows Google Maps download, not OSM!
There is a clause in the OpenStreetMap Tile usage policy which does not allow offline storage:


  Bulk downloading is strongly discouraged. Do not download tiles
  unnecessarily.
  
  In particular, downloading significant areas of tiles at zoom levels
  17 and higher for offline or later usage is forbidden without prior
  consultation with a System Administrator. These tiles are generally
  not available (cached) on the server in advance, and have to be
  rendered specifically for those requests, putting an unjustified
  burden on the available resources.
  ...


From Locus itself, you can also see that OSM is not downloadable, versus Google's (assumed to have limits, as mentioned in their TOS).