is there any particular reason why you installed it directly from source?
you should avoid doing that, as you cannot track the files you install: just build Slackware packages and install those.
another thing you can miss using directly the source, are (if not already present) some Slackware specific configuration files/scripts: in the openbox script from SBo
during building of the package, between the other things, a file /etc/X11/xinit/xinitrc.openbox is added to it (and it's later used by xwmconfig).
so, my suggestion is to remove the openbox you installed from source (try a "make uninstall" from the source folder) and install it using the openbox SlackBuild from SBo.