-- Goguen & Meseguer. Unifying Functional, Object-Oriented and Relational Programming with Logical Semantics. 1987
... object-oriented programming should be characterized by the nature of its classification mechanisms rather than by the nature of its communication mechanisms. Object-oriented programming is prescriptive in its methodology for classifying objects but is permissive in its methodology for communication.
object-oriented = objects + object classes + class inheritance
-- Wegner. The Object-Oriented Classification Paradigm. 1987