In a lot of ways, object oriented programming has been with the Mac from the start. The old-fashioned resources that were stored in a file's resource fork were objects; the idea was that you could modify those objects (which often represented things like user interface elements) without touching any of the code associated with them.
A big part of modern code design is separating the user interface from the logic. In the bad old days, procedures would write to the user interface directly, placing text into a window or modifying a dialog's appearance or function; nowadays, (good) programmers strive to separate the code's procedural functionality from its user interface as much as they can.
