If know a language like C++/Java/C#/JavaScript/etc, then try building a simple app without using any of the following constructs:

    if, while, for, foreach, do, goto, and switch. 

Also, don't expose any instance variables or use setters, and try to limit the use of assignment statements to constructors only.

Implementing something as simple as a FizzBuzz solution under those constraints will be enough to understand OOP.