Apple has their own object oriented programming guide that would be helpful.
http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/OOP_ObjC/Introduction/Introduction.html
Back in my first programming class in college i found a link to sun's oo guide, and found it very useful in understanding the concepts:
http://java.sun.com/docs/books/tutorial/java/concepts/
