Here are a couple of super-lame graphics to illustrate the difference between separating programmers in a large project using OO isolation mechanisms, vs letting everyone know and hack on a single data structure (using code review and testing for protection rather than isolation).

The circles are programs or portions of a program, and the squares are programmers.

First, OO:



Next, better:



In the OO case, you have portions of the program that are hard-separated (separated via the OO system). It is difficult to change the interactions, even if it is wrong. And the communication necessary to do so is expensive.

In the better case, the program is more monolithic, but has well understood data structures. Everybody hacks on the same code base (their own copy of it, of course). (Functions, files and namespaces may be used to hierarchically separate portions.) Each programmer submits his changes to a code-reviewer (more senior programmer). If he is interacting correctly with the data structures, his changes are incorporated. The project can organically become whatever it needs to become.