There's a customization group defined, Oop, that's been in there since  
1997, but at least in my build it is empty.  I couldn't find anything  
refer to it.

cus-edit.el:

(defgroup oop nil
   "Support for object-oriented programming."
   :group 'programming)


Also, I don't think the structure of these groups is very well though  
out: We have "Languages", then "Tools", which is helpfully labeled  
"Programming tools".  What's a tool, and what's not?  I think this  
means "general utilities: tools for programming in any language".
But the structuring isn't good in the first place.  Discounting the  
empty Oop group (which shouldn't be here anyways, since we lack a  
group for functional or imperative or whatever programming), there are  
just two entries in the "Programming" group.  This doesn't justify an  
extra level in the hierarchy.