gregorami,

For a comprehensive online OOP tutorial, you might check out Richard Baldwin (e.g. http://enos.itcollege.ee/~jpoial/oop/lugemist/aboutoop.html#an%20initial%20description%20of%20oop), Although the tutorials are technically Java based, they provide a solid grounding in basic OOP theory and are pretty clearly written.

Sun also has a good OOP (Java based again) tutorial: http://java.sun.com/docs/books/tutorial/java/concepts/

For a more purely Processing approach (please excuse the shameless plug) I devote a full chapter in my book (ch eight) to basic OOP with Processing.

There are also 2 (free to download!) chapters that may be of interest.
Chapter 14: 3D Rendering in Java Mode
Appendix C: Integrating Processing within Java
http://www.friendsofed.com/book.html?isbn=159059617X

Dan Shiffman also has some good online info on OOP (using Processing!), as part of his Introduction to Computational Media course: http://itp.nyu.edu/icm/shiffman/week3/index.html

There's also of course plenty of usful OOP info throughout this board.

Ira