/usr/include/OGRE/Plugins/OctreeSceneManager/ is in libogre-dev 1.7.4-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreHeightmapTerrainPageSource.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreOctree.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreOctreeCamera.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreOctreeNode.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreOctreePlugin.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreOctreeSceneManager.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreOctreeSceneQuery.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreTerrainPage.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreTerrainPageSource.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreTerrainPrerequisites.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreTerrainRenderable.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreTerrainSceneManager.h
/usr/include/OGRE/Plugins/OctreeSceneManager/OgreTerrainVertexProgram.h