Object-Oriented Programming is quite simple: it’s just choosing what function to run based on the parameters to the function (whether through method sending like Smalltalk, polymorphic lookup like CLOS, or table searching like C++: usually pattern-matching like Haskell would be excluded here).
Object-Oriented Analysis and Design is the thing where we represent our problem domain, and our solution, as a collection of objects, often categorised into classes, where the classes have particular relationships, properties and behaviours. And that is the thing that programmers often struggle with.
Whether it’s hard because OOA/D is hard, or because the problem domains are hard, or because OOA/D is not applicable to the problem domains, is not addressed here.
