Object orientation has many meanings to many people. To understand the concept one must see how object orientation is implemented or is an extension to your language.
object-oriented-programming : To understand the purpose behind object orientation you must first understand what it is.
object-systems: Libraries that add object oriented features to scheme (Varies by System).
CLOS The Common Lisp Object System that forms the basis of many scheme object systems
tiny-clos A object system that is based upon CLOS for Scheme.
YASOS A object system based upon T's object system.
meta-object-protocol A method for accessing the guts of an object system through a Meta Class.