The aim of this study is to reveal how well online communities of practice (oCoPs) help teachers share explicit knowledge and bring their tacit knowledge to the surface. An Internet based oCoP platform called "The Professional Development Circle" (The PDC) was developed for this study. The study was conducted in two phases: 1) a mandatory participation term and 2) a voluntary participation term. In the first phase, the researchers designed a learning environment in which 28 preservice teachers from three universities watched and discussed digital videos recorded in real classroom environments. In the second phase, the researchers tried to build another oCoP environment based on the successes and failures of the mandatory term but with voluntary members. This article presents the results to designers and educators who want to use oCoP environments in teachers' professional development. (Contains 1 figure.)