The Extern subsystem consists of libraries that are not part of the OpenMW project, but of which a copy is kept in the repository anyway:
Fork of the OIS Input Control System project, a library that makes it easy to handle input configurations for mouse, keyboard and joysticks by routing bindings through abstract channels. We use it for OpenMW's in-game key binding menu. When OpenMW switched to SDL2 input, modifications to oics were made to use SDL2 for input in place of OIS.
The abstract design of oics made it relatively painless for OpenMW to start supporting joysticks.
A port of the ogre-ffmpeg-videoplayer project, which is now in extern/ for historical reasons. We might move it to components/ at some point.