In Common Lisp, the object oriented system is called CLOS (Common Lisp Object System).

It is part of the Common Lisp ANSI standard.

It offers all advanced object oriented capabilities:

multi-inheritance, multi-methods, etc.

and last but not least, the meta-object protocol that gives users the ability to modify the language's behaviour and implementation (the benefit of this is that programmers can portably tailor CLOS to meet their specific programming needs, or even create their own Object Oriented language).

CLOS is a dynamic object oriented language.

This means that you can modify classes and objects on the fly, without stopping your application and without having to recompile everything.