Unfortunately, NomadBSD doesn’t have Openbox anymore.
It now uses the XFCE desktop environment. It’s not as big as GNOME or KDE, but it’s bigger than Openbox (which is just a window manager).
NomadBSD 132R is the last version supporting Openbox. It’s not available anymore from the main website/mirrors, but I saved a downloadable image here: Where to download 132R and 131R - #6 by gmacar