It's nearly impossible to write a Common Lisp program without using CLOS but most people don't know it.
For example, the different number types (integer, float, etc.) are system classes and nearly all math functions are generic functions that specialize their arguments on the number classes and then internally call the respective methods. That's how the "type contagion" (automatic type conversion according to the arguments) works.
This means that if you write (+ 1 1), you already have written an OOP program.
What makes CLOS different from other OOP systems is that classes and methods are two completely separate things that are not even necessarily related to each other. There can be classes without methods and methods without classes. It's up to the programmer to decide what makes sense.
The best CLOS tutorials I know are:
This list is surely incomplete and whoever sees that his/hers favourite CLOS tutorial is missing, may please leave a message here.
The standard state-of-the-art CLOS book is:
The most advanced CLOS book is:
The Meta Object Protocol
(MOP) is a de-facto standard, but not contained in the ANSI specification. You only need the MOP if you're planning to redefine classes at run-time and other funny things.
If you come from Java then I recommend Practical Common Lisp
for learning Lisp. It's written by Peter Seibel, who is a Java and Lisp programmer (and some other languages).