There has been little understanding of how the existence and exercise of formal intellectual property rights (IPR) such as patents influence the direction of OSS innovation. This dissertation seeks to bridge this gap in prior literature by focusing on two closely related topics. First, it investigates how OSS adoption and production are influenced by IPR enforcement exercised by proprietary incumbents. It suggests that when an IPR enforcement action is filed, user interest and developer activity will be negatively affected in two types of related OSS projects--those that display technology overlap with the litigated OSS and business projects that are specific to a focal litigated platform. The empirical analyses based on data from SourceForge.net strongly support the hypotheses. Second, it examines the impact of royalty-free patent pools contributed by OSS-friendly incumbents on OSS product entry by start-up firms. It argues that increases in the size of the OSS patent pool related to a software segment will facilitate OSS entry by start-up firms into the same segment; further, the marginal effect of the pool on OSS entry will be especially large in software segments where the cumulativeness of innovation is high or where patent ownership in a segment is concentrated. These hypotheses are empirically tested through examining the impacts of a major OSS patent pool--the Patent Commons, established by IBM and a few others in 2005--on OSS entry by 2,054 start-up firms from 1999 to 2009. The empirical results largely support these hypotheses and are robust to adding a variety of controls as well as to GMM instrumental variables estimation.