Object Oriented Design is a design "paradigm", or design "approach" that evolved from "data centric" design.
This means that rather than considering a system as a bunch of procedures that pass around and update various values - Procedural Programming - the system is considered as composed of a number of different Objects which have operations associated with them.
This is a piece of OO jargon that simply means that the operation invoked on an object is dependent on the type of the Object.
This builds on ideas of Specialization and Generalization in how Object Design is applied.
Object Oriented Design is directly supported with the Generic Persistent Object model of Cut The Crap Software.
This is taken a few stages further by The Alchemist that applies evolved Design Patterns to generate complete object models and web applications from a simple description.
Please check out the main website for more information and free downloads.