There are tons of sites out there that describe and talk about Object Oriented Programming. My guess is that there are so many sites because there are many different ways to learn the same thing. Some people learn from one method better than another.
Learning OOP
The best way to learn OOP is to read it and do it. Create things. It will cement the concepts and give you better recollection of when and why to use them.
| Part | Title | What You Will Learn About |
I
|
Overview
|
Learn about Objects, Classes, and OOP in general. |
II
|
Abstraction
|
How to recognize common object features and to centralize them. |
III
|
Encapsulation |
Isolating tasks and data through objects. |
IV
|
Inheritance
|
Leverage existing classes and their methods for similar and subclasses. |
V
|
Polymorphism |
Leveraging existing classes as "templates". |
You can find tons of additional resources using Google search and the terms OOP and C#, eBooks through Amazon, etc.
Articles / Concepts
Object-Oriented Programming (C# and Visual Basic) - MSDN
Object Oriented Programming Concepts - Black Wasp
Sample Code Projects
Object-Oriented Programming in C# .NET - Part 1 (Code Project)
Object-Oriented Programming in C# .NET - Part 2 (Code Project)