Do you want to transfer a few files occasionally to OSMC or do you want to be using Kodi GUI to browse files that are on your Raspbian system?
If the latter then you could install FileZilla SERVER on Raspbian. Then it acts as FTP server for your OSMC Kodi to access/browse/play.
If you want to transfer the occasional file to your Kodi system then install the OSMC FTP server (via Services in MyOSMC) and use FileZilla CLIENT on Raspbian to push the files across.
If the server is ob your Raspbian box then Id/pass is probably pi/raspberry (and you should change the password)