Get a book on design patterns. The Gang of Four book is a classic, and many people's bible, but the book is somewhat meaty and not very accessible. There are many simpler books available which explain the patterns in more user friendly language, but most of these have set opinions about one or more favourite patterns, and why they are so important.

An understanding of design patterns will help differentiate between good OOD and bad OOD, and also highlight when OO is appropriate.