Mainly there are four concepts for OOAD.
1.Abstraction -grouping of data and behaviour(operations) 
into a conceptual object.
2.Encapsulation -Its the implementation mode of data 
abstraction.With encapsulation, we can accomplish data 
hiding
3.Polymorphism -Polymorphism refers to the process whereby 
an object invokes a method of another object in a common 
manner (with the same name) without understanding or caring 
how it is accomplished. 

4.Inheritance-When an object is derived from another 
object. It inherits all the properties etc