Object-Oriented paradigm is a common paradigm for introductory programming courses. However, many teachers find that transitioning to teaching this paradigm is a difficult task. To overcome this complexity, many experienced teachers use microworlds to give beginner students an intuitive and rapid understanding of fundamental abstract concepts of Object-Oriented Programming, leading to a more effective learning. Microworlds have thus, become known to be engaging and facilitating complex concepts understanding, however, we know little on the process by which microworlds can be effective for learning, and how they motivate beginners. In this article, we attempt to identify the design principles of microworlds that offer support structures for providing engaging and meaningful learning activities that facilitate programming learning. We will explore individually each design principle, while focusing on its relation to learning and its relevance for learning Object-Oriented Programming. We finally, present PrOgO, a novel programming microworld, we designed on the basis of existing microworlds.