Every object-oriented programming language and environment has a different perspective on what object-oriented means, how objects behave, and how programs might be structured. This document offers the Objective-C perspective.
Whilst looking something up in Xcode documentation I stumbled across Object-Oriented Programming with Objective-C. It's a very readable introduction to OOP programming concepts coming in at just 38 pages covering:
- Object-Oriented Programming
- The Object Model
- Structuring Programs
- Structuring the Programming Task
Most enlightening are two final chapters discussing topics around clean OOP design and architecture.