Simula(67) definitely was the original concept/implementation of OO. Smalltalk, C++, Java, etc all derived different aspects of it.

- Main abstraction are classes (of objects).

- Concept of self-initializing data/procedure objects Internal ("concrete") view of an object vs an external ("abstract")

- Differentiated between object instances and the class

- Class/subclass facility made it possible to define generalized object classes, which could be specialized by defining subclasses containing additional declared properties

- Different subclasses could contain different virtual procedure declarations

- Domain specific language dialects