The important role of social capital has been extensively researched in real-world contexts. Much less attention has been paid to its relevance in the virtual world. Two fundamental questions need to be addressed: First, does there exist any online social capital in virtual world and how to measure it if it does exist? Second, how does online social capital affect the performance of virtual teams? In this study, we compare online and offline social networks and propose that comembership among virtual teams is the key mechanism for building online social networks, from which online social capital can be derived and appropriated. We further argue that the effectiveness of social capital depends not only on the number of ties a virtual team maintains with other teams, but also depends on the status of the members initiating the ties and to which other teams the ties are connected. In addition, we propose that online social capital can substitute team capability in affecting team performance. We test our framework using the open source software (OSS) development dataset. The results strongly support our framework, and bear important implications on the successful performance of virtual teams.