/usr/include/openbox/ is in openbox-dev 3.6.1-1ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/openbox/3.6/
/usr/include/openbox/3.6/obrender/
/usr/include/openbox/3.6/obrender/color.h
/usr/include/openbox/3.6/obrender/font.h
/usr/include/openbox/3.6/obrender/geom.h
/usr/include/openbox/3.6/obrender/gradient.h
/usr/include/openbox/3.6/obrender/image.h
/usr/include/openbox/3.6/obrender/instance.h
/usr/include/openbox/3.6/obrender/mask.h
/usr/include/openbox/3.6/obrender/render.h
/usr/include/openbox/3.6/obrender/theme.h
/usr/include/openbox/3.6/obrender/version.h
/usr/include/openbox/3.6/obt/
/usr/include/openbox/3.6/obt/display.h
/usr/include/openbox/3.6/obt/keyboard.h
/usr/include/openbox/3.6/obt/link.h
/usr/include/openbox/3.6/obt/paths.h
/usr/include/openbox/3.6/obt/prop.h
/usr/include/openbox/3.6/obt/signal.h
/usr/include/openbox/3.6/obt/util.h
/usr/include/openbox/3.6/obt/version.h
/usr/include/openbox/3.6/obt/xml.h
/usr/include/openbox/3.6/obt/xqueue.h