Separation of Concerns, Modularization, Reusability, Type Hierarchies, Type Composition, Interface contract-based programming, Frameworks etc. were all made mainstream by OOD/OOP. These are things taken for granted by programmers today. As somebody who has been doing OOD/OOP since the early nineties i can tell you it was the single biggest reason for the explosion of Software in the past few decades.

As a concrete example, early in my career i had programmed in C using the Windows API; both 16-bit and Win32 (Thank you Charles Petzold). It was difficult, tedious and a lot of work. And then Microsoft introduced MFC (Microsoft Foundation Classes) Framework with Visual C++ IDE. With a few clicks of the wizard, i had a complete skeleton application with a lot of hard work already done for you. That was a revelation for me on the power of OOD/OOP. Things i had slaved over in Win32 was now at the fingertips of every noob who could type. The same revelation happened (but not to the same extent) when i moved from Xlib to Motif on Unix platforms.

I had pointed you to Bertrand Meyer's book OOSC2 (in my other comment) as the book to read to understand OOD/OOP. Another great book to study is Barbara Liskov and John Guttag's Program Development in Java: Abstraction, Specification, and Object-Oriented Design.