Here’s a short list of themes around OOP and Ruby that I would recommend:

Practicing Ruby - Building Enumerable and Enumerator in Ruby
Makes a very well founded, simplificated introduction in Enumerable and Enumerator with great examples - concentrating on the major points.

Binstock on Software - Perfecting OO’s Small Classes and Short Methods
Runs you through really hard restrictions to look upon your classes.

Goodbye, shitty Car extends Vehicle object-orientation tutorial
A good post about not using “Real World Objects” for explaining OOP and polymorphism.

Test-Driven Development. A Cognitive Justification? - Steve Freeman
A critical and still valid view on TDD and Pairing.