Here is also a great free reference using Java:
http://docs.oracle.com/javase/tutorial/java/concepts/
Some short tips:
*An object consists of variables AND the object methods
*Most common methods are getters and setters
*Variables/Methods can have different visibilities, most common public/private
*In most cases OOP would be implemented by using class
*Although C is not OO, you can implement pseudo objects with the use of structs
*Objects can also inherit it's structure from a parent object
*There are several types of inheritance such as
https://en.wikipedia.org/wiki/Polymo...puter_science)
*Pros: Portable and reusable
*Cons: Can be robust