MobileOrg (the extension to org-mode in Emacs) requires an index.org file that points at the org files you want to export. You configure it to dump out anything from that index to a specified directory.

You then use something to sync that file tree to any of your devices. I used BTSync for a bit but found it annoying and I now use Google Drive with a GDrive syncing client on my phone so that the mobile-org Android app is always up-to-date.

It's bi-directional and they use a checksum matching scheme and some import tooling that allows you to suck in changes to those index files from other clients.