Whoever you are explaining this to needs to know what basic programming is before he can learn what OOP means, as it's a branch of programming languages. He can never understand what makes OOP special if he doesn't know its counterparts. So your question has two parts; how to explain what a programming language is and what separates OOP from other programming languages.
The easiest way to explain him what programming in general is about, is compare it to math operations. You can explain it by defining programming as a number of math expressions taking an input which results in an output. How far you wish to elaborate is up to you.
With this explanation we've done the ground work to make him understand what OOP means. Now we can define Objects as sets of mathematical functions and data. So instead of treating the logic as global chunks of code, we gather these chunks of code inside objects to gather relevant code pieces and get a way to isolate them. From this point on you can explain more advantages that come with the Object abstraction (e.g. polymorphism, loose coupling).
